diff --git a/api/index.php b/api/index.php
index 27c6af5..62029d3 100644
--- a/api/index.php
+++ b/api/index.php
@@ -55,7 +55,6 @@ $routes = [
     'backup'             => __DIR__ . '/../handlers/backup.php',
     'dev'                => __DIR__ . '/../handlers/dev.php',
     'dev/import'         => __DIR__ . '/../handlers/dev.php',
-    'health'             => __DIR__ . '/../handlers/health.php',
     'activity-log'       => __DIR__ . '/../handlers/activity_log.php',
 ];
 
diff --git a/common.php b/common.php
index 09d8395..7657791 100644
--- a/common.php
+++ b/common.php
@@ -235,23 +235,14 @@ function get_bearer_token(): ?string {
 function require_valid_token(): array {
     $token = get_bearer_token();
     if (!$token) {
-        http_response_code(401);
-        header('Content-Type: application/json; charset=UTF-8');
-        echo json_encode(["error" => "Missing Bearer token"]);
-        exit;
+        json_error('UNAUTHORIZED', 'Missing Bearer token', 401);
     }
     $rec = token_get_record($token);
     if (!$rec) {
-        http_response_code(403);
-        header('Content-Type: application/json; charset=UTF-8');
-        echo json_encode(["error" => "Invalid or expired token"]);
-        exit;
+        json_error('UNAUTHORIZED', 'Invalid or expired token', 401);
     }
     if (!empty($rec['temp'])) {
-        http_response_code(403);
-        header('Content-Type: application/json; charset=UTF-8');
-        echo json_encode(["error" => "Password change required before access"]);
-        exit;
+        json_error('PASSWORD_CHANGE_REQUIRED', 'Password change required before access', 403);
     }
     $rec['_token'] = $token;
     return $rec;
@@ -260,10 +251,7 @@ function require_valid_token(): array {
 function require_role(array $allowed, array $tokenRecord): void {
     $role = $tokenRecord['role'] ?? '';
     if (!in_array($role, $allowed, true)) {
-        http_response_code(403);
-        header('Content-Type: application/json; charset=UTF-8');
-        echo json_encode(["error" => "Insufficient permissions"]);
-        exit;
+        json_error('FORBIDDEN', 'Insufficient permissions', 403);
     }
 }
 
diff --git a/db_init.php b/db_init.php
index c3ab68e..7730ab9 100644
--- a/db_init.php
+++ b/db_init.php
@@ -283,3 +283,39 @@ function qdb_discard(string $tmpDb, $lockFp): void {
         @fclose($lockFp);
     }
 }
+
+/**
+ * Open a read-only DB connection; on failure logs and returns a JSON error response.
+ *
+ * @return array{0: PDO, 1: string, 2: resource|null}|null
+ */
+function qdb_open_read_or_fail(): ?array {
+    try {
+        return qdb_open(false);
+    } catch (Throwable $e) {
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/lib/errors.php';
+        error_log('qdb_open_read: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Database read'), 500);
+    }
+}
+
+/**
+ * Open a writable DB connection; on failure logs and returns a JSON error response.
+ *
+ * @return array{0: PDO, 1: string, 2: resource|null}|null
+ */
+function qdb_open_write_or_fail(): ?array {
+    try {
+        return qdb_open(true);
+    } catch (Throwable $e) {
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/lib/errors.php';
+        error_log('qdb_open_write: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Database write'), 500);
+    }
+}
diff --git a/docs/android-questionnaire-api.md b/docs/android-questionnaire-api.md
index 2b5ef31..b19d7ec 100644
--- a/docs/android-questionnaire-api.md
+++ b/docs/android-questionnaire-api.md
@@ -31,7 +31,34 @@ Use the front-controller routes under `/api/...`. The clean route `/api/app_ques
 }
 ```
 
-Some low-level auth failures can still return a legacy body such as `{"error":"Missing Bearer token"}` or `{"error":"Invalid or expired token"}`.
+Auth and permission failures use the same envelope, for example:
+
+```json
+{
+  "ok": false,
+  "error": {
+    "code": "UNAUTHORIZED",
+    "message": "Invalid or expired token"
+  }
+}
+```
+
+Submit validation failures (`POST /api/app_questionnaires`) return `code` `VALIDATION_FAILED` with a `details.errors` array:
+
+```json
+{
+  "ok": false,
+  "error": {
+    "code": "VALIDATION_FAILED",
+    "message": "Some answers are invalid or missing",
+    "details": {
+      "errors": [
+        { "questionID": "q3", "code": "MISSING_ANSWER", "message": "Required question is not answered" }
+      ]
+    }
+  }
+}
+```
 
 ## Authentication
 
diff --git a/handlers/activity_log.php b/handlers/activity_log.php
index d86c78b..8288664 100644
--- a/handlers/activity_log.php
+++ b/handlers/activity_log.php
@@ -13,8 +13,9 @@ if ($method !== 'GET') {
     json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
 }
 
-$date     = trim((string)($_GET['date'] ?? date('Y-m-d')));
-$activity = trim((string)($_GET['activity'] ?? ''));
-$limit    = (int)($_GET['limit'] ?? 200);
+$date       = trim((string)($_GET['date'] ?? date('Y-m-d')));
+$activity   = trim((string)($_GET['activity'] ?? ''));
+$limit      = (int)($_GET['limit'] ?? 200);
+$errorsOnly = !empty($_GET['errorsOnly']) || $activity === 'errors';
 
-json_success(qdb_api_log_read_entries($date, $activity, $limit));
+json_success(qdb_api_log_read_entries($date, $activity, $limit, $errorsOnly));
diff --git a/handlers/analytics.php b/handlers/analytics.php
index 2d10b23..267290d 100644
--- a/handlers/analytics.php
+++ b/handlers/analytics.php
@@ -8,8 +8,9 @@ require_once __DIR__ . '/../lib/analytics.php';
 switch ($method) {
 
 case 'GET':
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
     try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
         if (!empty($_GET['overview'])) {
             $data = qdb_analytics_overview($pdo, $tokenRec);
             qdb_discard($tmpDb, $lockFp);
@@ -25,9 +26,15 @@ case 'GET':
         qdb_discard($tmpDb, $lockFp);
         json_error('BAD_REQUEST', 'Unknown query', 400);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
         error_log('analytics GET: ' . $e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Analytics'), 500);
     }
     break;
 
@@ -36,15 +43,22 @@ case 'PUT':
     $clientCode = trim((string)($body['clientCode'] ?? ''));
     $note = (string)($body['note'] ?? '');
 
-    [$pdo, $tmpDb, $lockFp] = qdb_open(true);
     try {
+        $opened = qdb_open_write_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
         qdb_analytics_set_followup_note($pdo, $tokenRec, $clientCode, $note);
         qdb_save($tmpDb, $lockFp);
         json_success(['clientCode' => $clientCode, 'note' => $note]);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
         error_log('analytics PUT: ' . $e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Save note'), 500);
     }
     break;
 
diff --git a/handlers/answer_options.php b/handlers/answer_options.php
index d823409..8befcdf 100644
--- a/handlers/answer_options.php
+++ b/handlers/answer_options.php
@@ -10,22 +10,35 @@ case 'GET':
     if (!$qID) {
         json_error('MISSING_PARAM', 'questionID query param required', 400);
     }
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-    $stmt = $pdo->prepare('SELECT answerOptionID, questionID, defaultText, points, orderIndex, nextQuestionId FROM answer_option WHERE questionID = :qid ORDER BY orderIndex');
-    $stmt->execute([':qid' => $qID]);
-    $options = $stmt->fetchAll(PDO::FETCH_ASSOC);
-    foreach ($options as &$o) {
-        $o['points']     = (int)$o['points'];
-        $o['orderIndex'] = (int)$o['orderIndex'];
-        $o['optionKey'] = qdb_option_key($o);
-        $o['labelGerman'] = qdb_option_german_label($pdo, $o['answerOptionID'], $o['defaultText']);
-        $tr = $pdo->prepare('SELECT languageCode, text FROM answer_option_translation WHERE answerOptionID = :id');
-        $tr->execute([':id' => $o['answerOptionID']]);
-        $o['translations'] = $tr->fetchAll(PDO::FETCH_ASSOC);
+    try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        $stmt = $pdo->prepare('SELECT answerOptionID, questionID, defaultText, points, orderIndex, nextQuestionId FROM answer_option WHERE questionID = :qid ORDER BY orderIndex');
+        $stmt->execute([':qid' => $qID]);
+        $options = $stmt->fetchAll(PDO::FETCH_ASSOC);
+        foreach ($options as &$o) {
+            $o['points']     = (int)$o['points'];
+            $o['orderIndex'] = (int)$o['orderIndex'];
+            $o['optionKey'] = qdb_option_key($o);
+            $o['labelGerman'] = qdb_option_german_label($pdo, $o['answerOptionID'], $o['defaultText']);
+            $tr = $pdo->prepare('SELECT languageCode, text FROM answer_option_translation WHERE answerOptionID = :id');
+            $tr->execute([':id' => $o['answerOptionID']]);
+            $o['translations'] = $tr->fetchAll(PDO::FETCH_ASSOC);
+        }
+        unset($o);
+        qdb_discard($tmpDb, $lockFp);
+        json_success(['answerOptions' => $options]);
+    } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('answer_options GET: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load answer options'), 500);
     }
-    unset($o);
-    qdb_discard($tmpDb, $lockFp);
-    json_success(['answerOptions' => $options]);
     break;
 
 case 'POST':
diff --git a/handlers/app_questionnaires.php b/handlers/app_questionnaires.php
index b1f1c2a..dac8c02 100644
--- a/handlers/app_questionnaires.php
+++ b/handlers/app_questionnaires.php
@@ -13,7 +13,8 @@
 
 // Public app UI catalog for the login screen (no token, no PII).
 if ($method === 'GET' && !empty($_GET['translations'])) {
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+    $opened = qdb_open_read_or_fail();
+    [$pdo, $tmpDb, $lockFp] = $opened;
     json_success(['translations' => qdb_build_app_translations_map($pdo)]);
     qdb_discard($tmpDb, $lockFp);
     return;
@@ -39,7 +40,8 @@ if ($method === 'POST') {
     $completedAt = isset($body['completedAt']) ? (int)$body['completedAt'] : null;
 
     try {
-        [$pdo, $tmpDb, $lockFp] = qdb_open(true);
+        $opened = qdb_open_write_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
 
         // Verify questionnaire exists
         $qnStmt = $pdo->prepare("SELECT questionnaireID FROM questionnaire WHERE questionnaireID = :id");
@@ -101,6 +103,26 @@ if ($method === 'POST') {
             ];
         }
 
+        require_once __DIR__ . '/../lib/app_submit_validate.php';
+        $validationErrors = qdb_validate_app_submit_payload(
+            $pdo,
+            $qnID,
+            $answers,
+            $shortIdMap,
+            $shortIdToType,
+            $symptomParentMap,
+            $optionMap
+        );
+        if ($validationErrors !== []) {
+            qdb_discard($tmpDb, $lockFp);
+            json_error(
+                'VALIDATION_FAILED',
+                'Some answers are invalid or missing',
+                400,
+                ['errors' => $validationErrors]
+            );
+        }
+
         $pdo->beginTransaction();
 
         $sumPoints = 0;
@@ -120,7 +142,9 @@ if ($method === 'POST') {
 
         foreach ($answers as $a) {
             $shortId = trim($a['questionID'] ?? '');
-            if ($shortId === '') continue;
+            if ($shortId === '') {
+                continue;
+            }
 
             $answeredAt = isset($a['answeredAt']) ? (int)$a['answeredAt'] : null;
 
@@ -136,7 +160,9 @@ if ($method === 'POST') {
 
             // Resolve short ID to full questionID
             $fullQID = $shortIdMap[$shortId] ?? null;
-            if ($fullQID === null) continue;
+            if ($fullQID === null) {
+                continue;
+            }
 
             $answerOptionID = null;
             $freeTextValue  = isset($a['freeTextValue'])  ? (string)$a['freeTextValue']  : null;
@@ -228,10 +254,18 @@ if ($method === 'POST') {
 
         json_success(['submitted' => true, 'sumPoints' => $sumPoints]);
     } catch (Throwable $e) {
-        if (isset($pdo) && $pdo->inTransaction()) $pdo->rollBack();
-        if (isset($tmpDb, $lockFp)) qdb_discard($tmpDb, $lockFp);
-        error_log($e->getMessage());
-        json_error('SERVER_ERROR', 'Internal server error', 500);
+        if (isset($pdo) && $pdo->inTransaction()) {
+            $pdo->rollBack();
+        }
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('app_questionnaires POST: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Submit'), 500);
     }
 }
 
@@ -239,7 +273,8 @@ if ($method !== 'GET') {
     json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
 }
 
-[$pdo, $tmpDb, $lockFp] = qdb_open(false);
+$opened = qdb_open_read_or_fail();
+[$pdo, $tmpDb, $lockFp] = $opened;
 
 $qnID         = $_GET['id'] ?? '';
 $translations = $_GET['translations'] ?? '';
diff --git a/handlers/assignments.php b/handlers/assignments.php
index 5ccc6f8..4846356 100644
--- a/handlers/assignments.php
+++ b/handlers/assignments.php
@@ -72,7 +72,7 @@ case 'POST':
         }
         if (!$chk->fetch()) {
             qdb_discard($tmpDb, $lockFp);
-            json_error('NOT_FOUND', 'Coach not found or not authorized', 400);
+            json_error('NOT_FOUND', 'Coach not found or not authorized', 404);
         }
 
         [$rbacClause, $rbacParams] = rbac_client_filter($tokenRec);
diff --git a/handlers/backup.php b/handlers/backup.php
index ddaa1a7..cff1050 100644
--- a/handlers/backup.php
+++ b/handlers/backup.php
@@ -3,6 +3,10 @@
 $tokenRec = require_valid_token_web();
 require_role(['admin'], $tokenRec);
 
+if ($method !== 'POST') {
+    json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
+}
+
 $dbPath    = QDB_PATH;
 $backupDir = __DIR__ . '/../uploads/backups';
 
diff --git a/handlers/clients.php b/handlers/clients.php
index 5c3f492..96dfc07 100644
--- a/handlers/clients.php
+++ b/handlers/clients.php
@@ -67,7 +67,7 @@ case 'POST':
         }
         if (!$chkCoach->fetch()) {
             qdb_discard($tmpDb, $lockFp);
-            json_error('NOT_FOUND', 'Coach not found or not authorized', 400);
+            json_error('NOT_FOUND', 'Coach not found or not authorized', 404);
         }
 
         $chk = $pdo->prepare("SELECT clientCode FROM client WHERE clientCode = :cc");
diff --git a/handlers/coaches.php b/handlers/coaches.php
index fa72e46..3f703e9 100644
--- a/handlers/coaches.php
+++ b/handlers/coaches.php
@@ -9,8 +9,9 @@ if ($method !== 'GET') {
     json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
 }
 
-[$pdo, $tmpDb, $lockFp] = qdb_open(false);
 try {
+    $opened = qdb_open_read_or_fail();
+    [$pdo, $tmpDb, $lockFp] = $opened;
     $coachID = trim((string)($_GET['coachID'] ?? ''));
     if ($coachID !== '' && !empty($_GET['recent'])) {
         $limit = isset($_GET['limit']) ? (int)$_GET['limit'] : 100;
@@ -24,7 +25,13 @@ try {
     qdb_discard($tmpDb, $lockFp);
     json_success(['coaches' => $coaches]);
 } catch (Throwable $e) {
-    qdb_discard($tmpDb, $lockFp);
+    if (isset($tmpDb, $lockFp)) {
+        qdb_discard($tmpDb, $lockFp);
+    }
+    if (function_exists('qdb_api_log_note_exception')) {
+        qdb_api_log_note_exception($e);
+    }
+    require_once __DIR__ . '/../lib/errors.php';
     error_log('coaches GET: ' . $e->getMessage());
-    json_error('SERVER_ERROR', 'Server error', 500);
+    json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load coaches'), 500);
 }
diff --git a/handlers/dev.php b/handlers/dev.php
index 3478dcd..dbf8ddd 100644
--- a/handlers/dev.php
+++ b/handlers/dev.php
@@ -53,5 +53,10 @@ try {
     ];
     $label = $labels[$action ?? 'import'] ?? 'Operation';
     error_log("dev fixture $label: " . $e->getMessage());
-    json_error('SERVER_ERROR', "$label failed: " . $e->getMessage(), 500);
+    require_once __DIR__ . '/../lib/errors.php';
+    if (function_exists('qdb_api_log_note_exception')) {
+        qdb_api_log_note_exception($e);
+    }
+    error_log("dev fixture $label: " . $e->getMessage());
+    json_error('SERVER_ERROR', qdb_public_error_message($e, $label), 500);
 }
diff --git a/handlers/export.php b/handlers/export.php
index 1512a37..2c0f749 100644
--- a/handlers/export.php
+++ b/handlers/export.php
@@ -10,20 +10,35 @@ if ($method !== 'GET') {
 
 if (!empty($_GET['bundle'])) {
     require_role(['admin', 'supervisor'], $tokenRec);
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+    try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
     $bundle = qdb_export_all_questionnaires_bundle($pdo);
     qdb_discard($tmpDb, $lockFp);
 
     $filename = 'questionnaires_bundle_' . date('Y-m-d_His') . '.json';
     header('Content-Type: application/json; charset=UTF-8');
     header('Content-Disposition: attachment; filename="' . $filename . '"');
-    echo json_encode($bundle, JSON_UNESCAPED_UNICODE | JSON_PRETTY_PRINT);
-    exit;
+        echo json_encode($bundle, JSON_UNESCAPED_UNICODE | JSON_PRETTY_PRINT);
+        exit;
+    } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('export bundle: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Export'), 500);
+    }
 }
 
 if (!empty($_GET['exportAll'])) {
     require_role(['admin'], $tokenRec);
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+    try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
     $allVersions = !empty($_GET['allVersions']);
     $zipPath = qdb_build_server_export_zip($pdo, $tokenRec, $allVersions);
     qdb_discard($tmpDb, $lockFp);
@@ -34,9 +49,20 @@ if (!empty($_GET['exportAll'])) {
     header('Content-Type: application/zip');
     header('Content-Disposition: attachment; filename="' . $filename . '"');
     header('Content-Length: ' . (string)filesize($zipPath));
-    readfile($zipPath);
-    @unlink($zipPath);
-    exit;
+        readfile($zipPath);
+        @unlink($zipPath);
+        exit;
+    } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('export exportAll: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Export'), 500);
+    }
 }
 
 $qnID = $_GET['questionnaireID'] ?? '';
@@ -46,7 +72,9 @@ if (!$qnID) {
 
 $allVersions = !empty($_GET['allVersions']);
 
-[$pdo, $tmpDb, $lockFp] = qdb_open(false);
+try {
+    $opened = qdb_open_read_or_fail();
+    [$pdo, $tmpDb, $lockFp] = $opened;
 
 $qn = $pdo->prepare('SELECT * FROM questionnaire WHERE questionnaireID = :id');
 $qn->execute([':id' => $qnID]);
@@ -80,6 +108,17 @@ qdb_discard($tmpDb, $lockFp);
 $safeName = qdb_export_safe_basename($questionnaire['name']);
 $filename = $safeName . '_v' . $questionnaire['version'] . '.csv';
 
-header('Content-Type: text/csv; charset=UTF-8');
-header('Content-Disposition: attachment; filename="' . $filename . '"');
-echo qdb_export_rows_to_csv_string($rows, qdb_export_current_csv_fallback_header($resultColumns));
+    header('Content-Type: text/csv; charset=UTF-8');
+    header('Content-Disposition: attachment; filename="' . $filename . '"');
+    echo qdb_export_rows_to_csv_string($rows, qdb_export_current_csv_fallback_header($resultColumns));
+} catch (Throwable $e) {
+    if (isset($tmpDb, $lockFp)) {
+        qdb_discard($tmpDb, $lockFp);
+    }
+    if (function_exists('qdb_api_log_note_exception')) {
+        qdb_api_log_note_exception($e);
+    }
+    require_once __DIR__ . '/../lib/errors.php';
+    error_log('export csv: ' . $e->getMessage());
+    json_error('SERVER_ERROR', qdb_public_error_message($e, 'Export'), 500);
+}
diff --git a/handlers/health.php b/handlers/health.php
deleted file mode 100644
index a840cb0..0000000
--- a/handlers/health.php
+++ /dev/null
@@ -1,42 +0,0 @@
-<?php
-/**
- * GET /api/health — verify FPM sees the same project root, key, and DB as CLI.
- */
-if ($method !== 'GET') {
-    json_error('METHOD_NOT_ALLOWED', 'Method not allowed', 405);
-}
-
-$dbOpenOk = false;
-$dbError  = null;
-try {
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-    qdb_discard($tmpDb, $lockFp);
-    $dbOpenOk = true;
-} catch (Throwable $e) {
-    $dbError = $e->getMessage();
-}
-
-$keySha = null;
-$keyError = null;
-try {
-    $keySha = hash('sha256', get_master_key_bytes());
-} catch (Throwable $e) {
-    $keyError = $e->getMessage();
-}
-
-json_success([
-    'sapi'          => PHP_SAPI,
-    'projectRoot'   => realpath(__DIR__ . '/..') ?: (__DIR__ . '/..'),
-    'commonFile'    => realpath(__DIR__ . '/../common.php') ?: (__DIR__ . '/../common.php'),
-    'commonMtime'   => @filemtime(__DIR__ . '/../common.php') ?: 0,
-    'envFile'       => qdb_env_file_path(),
-    'envReadable'   => is_readable(qdb_env_file_path()),
-    'keySha256'     => $keySha,
-    'keyError'      => $keyError,
-    'dbPath'        => QDB_PATH,
-    'dbRealpath'    => realpath(QDB_PATH) ?: null,
-    'dbBytes'       => is_file(QDB_PATH) ? filesize(QDB_PATH) : 0,
-    'dbOpenOk'      => $dbOpenOk,
-    'dbError'       => $dbError,
-    'keyMatchesCli' => $keySha === 'e584e16eb81fbb4f1e3aac965b5225c5fefabc2a67ea20e26e55671c7d9f1ae7',
-]);
diff --git a/handlers/logout.php b/handlers/logout.php
index dbd8611..3990214 100644
--- a/handlers/logout.php
+++ b/handlers/logout.php
@@ -9,5 +9,15 @@ if (!$token) {
     json_error('UNAUTHORIZED', 'Missing Bearer token', 401);
 }
 
-token_revoke($token);
+try {
+    token_revoke($token);
+} catch (Throwable $e) {
+    if (function_exists('qdb_api_log_note_exception')) {
+        qdb_api_log_note_exception($e);
+    }
+    require_once __DIR__ . '/../lib/errors.php';
+    error_log('logout: ' . $e->getMessage());
+    json_error('SERVER_ERROR', qdb_public_error_message($e, 'Logout'), 500);
+}
+
 json_success(['loggedOut' => true]);
diff --git a/handlers/questionnaires.php b/handlers/questionnaires.php
index 41c3c68..6d718ad 100644
--- a/handlers/questionnaires.php
+++ b/handlers/questionnaires.php
@@ -5,8 +5,9 @@ $tokenRec = require_valid_token_web();
 switch ($method) {
 
 case 'GET':
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
     try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
         [$rbacClause, $rbacParams] = rbac_client_filter($tokenRec, 'cl');
         $sql = "
             SELECT q.questionnaireID, q.name, q.version, q.state,
@@ -39,9 +40,15 @@ case 'GET':
         qdb_discard($tmpDb, $lockFp);
         json_success(['questionnaires' => $rows, 'categoryKeys' => $categoryKeys]);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
         error_log('questionnaires GET: ' . $e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load questionnaires'), 500);
     }
     break;
 
@@ -122,7 +129,12 @@ case 'POST':
         } catch (Throwable $e) {
             qdb_discard($tmpDb, $lockFp);
             error_log('importBundle: ' . $e->getMessage());
-            json_error('SERVER_ERROR', $e->getMessage(), 500);
+            require_once __DIR__ . '/../lib/errors.php';
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            error_log('importBundle: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Import'), 500);
         }
         break;
     }
diff --git a/handlers/questions.php b/handlers/questions.php
index ee22c05..f38b7a0 100644
--- a/handlers/questions.php
+++ b/handlers/questions.php
@@ -9,8 +9,10 @@ case 'GET':
     if (!$qnID) {
         json_error('BAD_REQUEST', 'questionnaireID query param required', 400);
     }
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-    $stmt = $pdo->prepare("
+    try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        $stmt = $pdo->prepare("
         SELECT questionID, questionnaireID, defaultText, type, orderIndex, isRequired, configJson
         FROM question WHERE questionnaireID = :qid ORDER BY orderIndex
     ");
@@ -40,9 +42,20 @@ case 'GET':
         $tr->execute([':qid' => $q['questionID']]);
         $q['translations'] = $tr->fetchAll(PDO::FETCH_ASSOC);
     }
-    unset($q);
-    qdb_discard($tmpDb, $lockFp);
-    json_success(['questions' => $questions]);
+        unset($q);
+        qdb_discard($tmpDb, $lockFp);
+        json_success(['questions' => $questions]);
+    } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('questions GET: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load questions'), 500);
+    }
     break;
 
 case 'POST':
@@ -204,18 +217,57 @@ case 'PATCH':
     if (empty($body['questionnaireID']) || !is_array($body['order'] ?? null)) {
         json_error('BAD_REQUEST', 'questionnaireID and order[] required', 400);
     }
-    [$pdo, $tmpDb, $lockFp] = qdb_open(true);
+    $qnID = (string)$body['questionnaireID'];
+    $orderIds = array_values(array_filter(
+        array_map('strval', $body['order']),
+        static fn($id) => $id !== ''
+    ));
+    if ($orderIds === []) {
+        json_error('BAD_REQUEST', 'order[] must list at least one questionID', 400);
+    }
     try {
-        $stmt = $pdo->prepare("UPDATE question SET orderIndex = :o WHERE questionID = :id AND questionnaireID = :qid");
-        foreach ($body['order'] as $idx => $qid) {
-            $stmt->execute([':o' => $idx, ':id' => $qid, ':qid' => $body['questionnaireID']]);
+        $opened = qdb_open_write_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        $chk = $pdo->prepare('SELECT 1 FROM questionnaire WHERE questionnaireID = :id');
+        $chk->execute([':id' => $qnID]);
+        if (!$chk->fetch()) {
+            qdb_discard($tmpDb, $lockFp);
+            json_error('NOT_FOUND', 'Questionnaire not found', 404);
+        }
+        $ph = implode(',', array_fill(0, count($orderIds), '?'));
+        $stmt = $pdo->prepare(
+            "SELECT questionID FROM question WHERE questionnaireID = ? AND questionID IN ($ph)"
+        );
+        $stmt->execute(array_merge([$qnID], $orderIds));
+        $found = $stmt->fetchAll(PDO::FETCH_COLUMN);
+        if (count($found) !== count($orderIds)) {
+            $missing = array_values(array_diff($orderIds, $found));
+            qdb_discard($tmpDb, $lockFp);
+            json_error(
+                'INVALID_FIELD',
+                'order[] contains questionIDs not in this questionnaire',
+                400,
+                ['missingQuestionIDs' => $missing]
+            );
+        }
+        $upd = $pdo->prepare(
+            'UPDATE question SET orderIndex = :o WHERE questionID = :id AND questionnaireID = :qid'
+        );
+        foreach ($orderIds as $idx => $qid) {
+            $upd->execute([':o' => $idx, ':id' => $qid, ':qid' => $qnID]);
         }
         qdb_save($tmpDb, $lockFp);
         json_success(['reordered' => true]);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
-        error_log($e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('questions PATCH: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Reorder questions'), 500);
     }
     break;
 
diff --git a/handlers/results.php b/handlers/results.php
index c50943f..3de8f5e 100644
--- a/handlers/results.php
+++ b/handlers/results.php
@@ -13,9 +13,11 @@ if (!$qnID) {
     json_error('MISSING_PARAM', 'questionnaireID query param required', 400);
 }
 
-[$pdo, $tmpDb, $lockFp] = qdb_open(false);
+try {
+    $opened = qdb_open_read_or_fail();
+    [$pdo, $tmpDb, $lockFp] = $opened;
 
-$qn = $pdo->prepare("SELECT * FROM questionnaire WHERE questionnaireID = :id");
+    $qn = $pdo->prepare("SELECT * FROM questionnaire WHERE questionnaireID = :id");
 $qn->execute([':id' => $qnID]);
 $questionnaire = $qn->fetch(PDO::FETCH_ASSOC);
 if (!$questionnaire) {
@@ -110,10 +112,21 @@ if (!empty($questionIDs) && !empty($clients)) {
     unset($c);
 }
 
-qdb_discard($tmpDb, $lockFp);
+    qdb_discard($tmpDb, $lockFp);
 
-json_success([
-    "questionnaire" => $questionnaire,
-    "questions"     => $questions,
-    "clients"       => $clients,
-]);
+    json_success([
+        'questionnaire' => $questionnaire,
+        'questions'     => $questions,
+        'clients'       => $clients,
+    ]);
+} catch (Throwable $e) {
+    if (isset($tmpDb, $lockFp)) {
+        qdb_discard($tmpDb, $lockFp);
+    }
+    if (function_exists('qdb_api_log_note_exception')) {
+        qdb_api_log_note_exception($e);
+    }
+    require_once __DIR__ . '/../lib/errors.php';
+    error_log('results GET: ' . $e->getMessage());
+    json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load results'), 500);
+}
diff --git a/handlers/session.php b/handlers/session.php
index 8aceb9a..7eebaad 100644
--- a/handlers/session.php
+++ b/handlers/session.php
@@ -26,14 +26,23 @@ if ($role === 'coach') {
 $username = '';
 if (($rec['userID'] ?? '') !== '') {
     try {
-        [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
         $stmt = $pdo->prepare('SELECT username FROM users WHERE userID = :uid');
         $stmt->execute([':uid' => $rec['userID']]);
         $row = $stmt->fetchColumn();
         $username = $row !== false ? (string)$row : '';
         qdb_discard($tmpDb, $lockFp);
     } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
         error_log('session username lookup: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Session check'), 500);
     }
 }
 
diff --git a/handlers/translations.php b/handlers/translations.php
index cb69047..b9b3320 100644
--- a/handlers/translations.php
+++ b/handlers/translations.php
@@ -9,9 +9,22 @@ switch ($method) {
 case 'GET':
     if (!empty($_GET['exportBundle'])) {
         require_role(['admin', 'supervisor'], $tokenRec);
-        [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-        $bundle = qdb_export_translations_bundle($pdo);
-        qdb_discard($tmpDb, $lockFp);
+        try {
+            $opened = qdb_open_read_or_fail();
+            [$pdo, $tmpDb, $lockFp] = $opened;
+            $bundle = qdb_export_translations_bundle($pdo);
+            qdb_discard($tmpDb, $lockFp);
+        } catch (Throwable $e) {
+            if (isset($tmpDb, $lockFp)) {
+                qdb_discard($tmpDb, $lockFp);
+            }
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            require_once __DIR__ . '/../lib/errors.php';
+            error_log('translations exportBundle: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Export'), 500);
+        }
 
         $filename = 'translations_bundle_' . date('Y-m-d_His') . '.json';
         header('Content-Type: application/json; charset=UTF-8');
@@ -21,14 +34,29 @@ case 'GET':
     }
 
     if (isset($_GET['languages'])) {
-        [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-        $rows = $pdo->query("SELECT languageCode, name FROM language ORDER BY languageCode")->fetchAll(PDO::FETCH_ASSOC);
-        qdb_discard($tmpDb, $lockFp);
-        json_success(["languages" => $rows]);
+        try {
+            $opened = qdb_open_read_or_fail();
+            [$pdo, $tmpDb, $lockFp] = $opened;
+            $rows = $pdo->query("SELECT languageCode, name FROM language ORDER BY languageCode")->fetchAll(PDO::FETCH_ASSOC);
+            qdb_discard($tmpDb, $lockFp);
+            json_success(['languages' => $rows]);
+        } catch (Throwable $e) {
+            if (isset($tmpDb, $lockFp)) {
+                qdb_discard($tmpDb, $lockFp);
+            }
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            require_once __DIR__ . '/../lib/errors.php';
+            error_log('translations languages: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load languages'), 500);
+        }
     }
 
     if (!empty($_GET['all'])) {
-        [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+        try {
+            $opened = qdb_open_read_or_fail();
+            [$pdo, $tmpDb, $lockFp] = $opened;
 
         $languages = $pdo->query("SELECT languageCode, name FROM language ORDER BY languageCode")->fetchAll(PDO::FETCH_ASSOC);
         if (!array_filter($languages, fn($l) => $l['languageCode'] === QDB_SOURCE_LANGUAGE)) {
@@ -65,41 +93,69 @@ case 'GET':
             }
         }
 
-        qdb_discard($tmpDb, $lockFp);
-        json_success([
-            'languages'       => $languages,
-            'appStrings'      => $appEntries,
-            'questionnaires'  => $questionnaires,
-            'entries'         => $allEntries,
-            'sourceLanguage'  => QDB_SOURCE_LANGUAGE,
-        ]);
+            qdb_discard($tmpDb, $lockFp);
+            json_success([
+                'languages'       => $languages,
+                'appStrings'      => $appEntries,
+                'questionnaires'  => $questionnaires,
+                'entries'         => $allEntries,
+                'sourceLanguage'  => QDB_SOURCE_LANGUAGE,
+            ]);
+        } catch (Throwable $e) {
+            if (isset($tmpDb, $lockFp)) {
+                qdb_discard($tmpDb, $lockFp);
+            }
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            require_once __DIR__ . '/../lib/errors.php';
+            error_log('translations all: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load translations'), 500);
+        }
     }
 
-    $qnID = $_GET['questionnaireID'] ?? '';
-    if ($qnID) {
-        [$pdo, $tmpDb, $lockFp] = qdb_open(false);
+    $qnID = trim((string)($_GET['questionnaireID'] ?? ''));
+    if ($qnID !== '') {
+        try {
+            $opened = qdb_open_read_or_fail();
+            [$pdo, $tmpDb, $lockFp] = $opened;
 
-        $languages = $pdo->query("SELECT languageCode, name FROM language ORDER BY languageCode")->fetchAll(PDO::FETCH_ASSOC);
-        if (!array_filter($languages, fn($l) => $l['languageCode'] === QDB_SOURCE_LANGUAGE)) {
-            array_unshift($languages, ['languageCode' => QDB_SOURCE_LANGUAGE, 'name' => 'German']);
-        }
+            $languages = $pdo->query("SELECT languageCode, name FROM language ORDER BY languageCode")->fetchAll(PDO::FETCH_ASSOC);
+            if (!array_filter($languages, fn($l) => $l['languageCode'] === QDB_SOURCE_LANGUAGE)) {
+                array_unshift($languages, ['languageCode' => QDB_SOURCE_LANGUAGE, 'name' => 'German']);
+            }
 
-        $qnRow = $pdo->prepare("SELECT name FROM questionnaire WHERE questionnaireID = :id");
-        $qnRow->execute([':id' => $qnID]);
-        $qnName = $qnRow->fetchColumn() ?: '';
+            $qnRow = $pdo->prepare('SELECT name FROM questionnaire WHERE questionnaireID = :id');
+            $qnRow->execute([':id' => $qnID]);
+            $qnName = $qnRow->fetchColumn();
+            if ($qnName === false) {
+                qdb_discard($tmpDb, $lockFp);
+                json_error('NOT_FOUND', 'Questionnaire not found', 404);
+            }
 
-        $lists = qdb_translation_entry_lists($pdo, $qnID);
+            $lists = qdb_translation_entry_lists($pdo, $qnID);
         $entries = array_merge($lists['stringEntries'], $lists['contentEntries']);
         $translations = qdb_load_translations_for_entries($pdo, $entries);
         qdb_attach_translation_texts($entries, $translations);
 
-        qdb_discard($tmpDb, $lockFp);
-        json_success([
-            'languages'      => $languages,
-            'entries'        => $entries,
-            'questionnaire'  => ['questionnaireID' => $qnID, 'name' => $qnName],
-            'sourceLanguage' => QDB_SOURCE_LANGUAGE,
-        ]);
+            qdb_discard($tmpDb, $lockFp);
+            json_success([
+                'languages'      => $languages,
+                'entries'        => $entries,
+                'questionnaire'  => ['questionnaireID' => $qnID, 'name' => (string)$qnName],
+                'sourceLanguage' => QDB_SOURCE_LANGUAGE,
+            ]);
+        } catch (Throwable $e) {
+            if (isset($tmpDb, $lockFp)) {
+                qdb_discard($tmpDb, $lockFp);
+            }
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            require_once __DIR__ . '/../lib/errors.php';
+            error_log('translations questionnaire: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load translations'), 500);
+        }
     }
 
     $type = $_GET['type'] ?? '';
@@ -108,8 +164,10 @@ case 'GET':
         json_error('BAD_REQUEST', 'type (question|answer_option|string) and id required', 400);
     }
 
-    [$pdo, $tmpDb, $lockFp] = qdb_open(false);
-    if ($type === 'question') {
+    try {
+        $opened = qdb_open_read_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        if ($type === 'question') {
         $stmt = $pdo->prepare("SELECT languageCode, text FROM question_translation WHERE questionID = :id");
         $stmt->execute([':id' => $id]);
     } elseif ($type === 'answer_option') {
@@ -123,9 +181,27 @@ case 'GET':
             $stmt = $pdo->query("SELECT stringKey, languageCode, text FROM string_translation ORDER BY stringKey, languageCode");
         }
     }
-    $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
-    qdb_discard($tmpDb, $lockFp);
-    json_success(["translations" => $rows]);
+        if ($type === 'question' || $type === 'answer_option') {
+            if (!qdb_translation_entity_exists($pdo, $type, (string)$id)) {
+                qdb_discard($tmpDb, $lockFp);
+                json_error('NOT_FOUND', 'Translation entity not found', 404);
+            }
+        }
+
+        $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
+        qdb_discard($tmpDb, $lockFp);
+        json_success(['translations' => $rows]);
+    } catch (Throwable $e) {
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('translations GET: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Load translations'), 500);
+    }
     break;
 
 case 'POST':
@@ -144,7 +220,12 @@ case 'POST':
         } catch (Throwable $e) {
             qdb_discard($tmpDb, $lockFp);
             error_log('translations importBundle: ' . $e->getMessage());
-            json_error('SERVER_ERROR', $e->getMessage(), 500);
+            require_once __DIR__ . '/../lib/errors.php';
+            if (function_exists('qdb_api_log_note_exception')) {
+                qdb_api_log_note_exception($e);
+            }
+            error_log('translations importBundle: ' . $e->getMessage());
+            json_error('SERVER_ERROR', qdb_public_error_message($e, 'Import'), 500);
         }
         break;
     }
@@ -188,15 +269,26 @@ case 'PUT':
         json_error('MISSING_FIELDS', 'type, id, and languageCode required', 400);
     }
 
-    [$pdo, $tmpDb, $lockFp] = qdb_open(true);
     try {
+        $opened = qdb_open_write_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        if (!qdb_translation_entity_exists($pdo, $type, (string)$id)) {
+            qdb_discard($tmpDb, $lockFp);
+            json_error('NOT_FOUND', 'Translation entity not found', 404);
+        }
         qdb_put_translation($pdo, $type, $id, $lang, $text);
         qdb_save($tmpDb, $lockFp);
         json_success([]);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
-        error_log($e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('translations PUT: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Save translation'), 500);
     }
     break;
 
@@ -235,24 +327,37 @@ case 'DELETE':
         json_error('MISSING_FIELDS', 'type, id, and languageCode required', 400);
     }
 
-    [$pdo, $tmpDb, $lockFp] = qdb_open(true);
     try {
+        $opened = qdb_open_write_or_fail();
+        [$pdo, $tmpDb, $lockFp] = $opened;
+        if ($type === 'question' || $type === 'answer_option') {
+            if (!qdb_translation_entity_exists($pdo, $type, (string)$id)) {
+                qdb_discard($tmpDb, $lockFp);
+                json_error('NOT_FOUND', 'Translation entity not found', 404);
+            }
+        }
         if ($type === 'question') {
-            $pdo->prepare("DELETE FROM question_translation WHERE questionID = :id AND languageCode = :lang")
+            $pdo->prepare('DELETE FROM question_translation WHERE questionID = :id AND languageCode = :lang')
                 ->execute([':id' => $id, ':lang' => $lang]);
         } elseif ($type === 'answer_option') {
-            $pdo->prepare("DELETE FROM answer_option_translation WHERE answerOptionID = :id AND languageCode = :lang")
+            $pdo->prepare('DELETE FROM answer_option_translation WHERE answerOptionID = :id AND languageCode = :lang')
                 ->execute([':id' => $id, ':lang' => $lang]);
         } else {
-            $pdo->prepare("DELETE FROM string_translation WHERE stringKey = :id AND languageCode = :lang")
+            $pdo->prepare('DELETE FROM string_translation WHERE stringKey = :id AND languageCode = :lang')
                 ->execute([':id' => $id, ':lang' => $lang]);
         }
         qdb_save($tmpDb, $lockFp);
         json_success([]);
     } catch (Throwable $e) {
-        qdb_discard($tmpDb, $lockFp);
-        error_log($e->getMessage());
-        json_error('SERVER_ERROR', 'Server error', 500);
+        if (isset($tmpDb, $lockFp)) {
+            qdb_discard($tmpDb, $lockFp);
+        }
+        if (function_exists('qdb_api_log_note_exception')) {
+            qdb_api_log_note_exception($e);
+        }
+        require_once __DIR__ . '/../lib/errors.php';
+        error_log('translations DELETE: ' . $e->getMessage());
+        json_error('SERVER_ERROR', qdb_public_error_message($e, 'Delete translation'), 500);
     }
     break;
 
diff --git a/lib/api_log.php b/lib/api_log.php
index ce8b22a..2272b6c 100644
--- a/lib/api_log.php
+++ b/lib/api_log.php
@@ -15,7 +15,7 @@
  */
 
 /** @var list<string> */
-const QDB_API_LOG_ACTIVITIES = ['app_sync', 'app_change', 'web_change', 'web_export'];
+const QDB_API_LOG_ACTIVITIES = ['app_sync', 'app_change', 'web_change', 'web_export', 'api_error'];
 
 function qdb_api_log_enabled(): bool
 {
@@ -150,7 +150,7 @@ function qdb_api_log_classify_activity(string $method, string $client, string $r
     $client = strtolower(trim($client));
     $route  = strtolower(trim($route));
 
-    if ($method === 'OPTIONS' || in_array($route, ['health', 'activity-log', 'session'], true)) {
+    if ($method === 'OPTIONS' || in_array($route, ['activity-log', 'session'], true)) {
         return null;
     }
 
@@ -191,37 +191,48 @@ function qdb_api_log_begin(string $method, string $route, array $context = []):
 
     $client   = trim((string)($_SERVER['HTTP_X_QDB_CLIENT'] ?? ''));
     $activity = qdb_api_log_classify_activity($method, $client, $route);
-    if ($activity === null) {
-        $GLOBALS['qdb_api_log_ctx'] = ['skip' => true];
-        return;
-    }
 
     $GLOBALS['qdb_api_log_ctx'] = array_merge([
         'started_at' => microtime(true),
         'method'     => strtoupper($method),
         'route'      => $route,
-        'activity'   => $activity,
         'uri'        => (string)($_SERVER['REQUEST_URI'] ?? ''),
         'ip'         => qdb_api_log_client_ip(),
         'client'     => $client,
         'user_agent' => trim((string)($_SERVER['HTTP_USER_AGENT'] ?? '')),
     ], $context);
+
+    if ($activity !== null) {
+        $GLOBALS['qdb_api_log_ctx']['activity'] = $activity;
+    } else {
+        $GLOBALS['qdb_api_log_ctx']['skip_success'] = true;
+    }
 }
 
 function qdb_api_log_note_exception(Throwable $e): void
 {
-    if (empty($GLOBALS['qdb_api_log_ctx']) || !empty($GLOBALS['qdb_api_log_ctx']['skip'])) {
+    if (empty($GLOBALS['qdb_api_log_ctx'])) {
         return;
     }
     $GLOBALS['qdb_api_log_ctx']['exception'] = $e->getMessage();
     $GLOBALS['qdb_api_log_ctx']['exception_class'] = $e::class;
 }
 
+function qdb_api_log_note_api_error(string $code, string $message, int $status): void
+{
+    if (empty($GLOBALS['qdb_api_log_ctx'])) {
+        return;
+    }
+    $GLOBALS['qdb_api_log_ctx']['api_error_code']    = $code;
+    $GLOBALS['qdb_api_log_ctx']['api_error_message'] = $message;
+    $GLOBALS['qdb_api_log_ctx']['api_error_status']  = $status;
+}
+
 function qdb_api_log_finish(): void
 {
     $ctx = $GLOBALS['qdb_api_log_ctx'] ?? null;
     unset($GLOBALS['qdb_api_log_ctx']);
-    if (!is_array($ctx) || !qdb_api_log_enabled() || !empty($ctx['skip'])) {
+    if (!is_array($ctx) || !qdb_api_log_enabled()) {
         return;
     }
 
@@ -231,6 +242,14 @@ function qdb_api_log_finish(): void
         $status = 200;
     }
 
+    $isError = $status >= 400
+        || !empty($ctx['exception'])
+        || !empty($ctx['api_error_code']);
+
+    if (!empty($ctx['skip_success']) && !$isError) {
+        return;
+    }
+
     $token = get_bearer_token();
     $auth  = ['role' => null, 'userID' => null, 'token_hint' => null];
     if ($token !== null && $token !== '') {
@@ -256,9 +275,14 @@ function qdb_api_log_finish(): void
     $actorUsername  = qdb_api_log_lookup_username($auth['userID'] ?? null);
     $targetUsername = qdb_api_log_extract_target_username($bodyLog, $queryLog);
 
+    $activity = $ctx['activity'] ?? null;
+    if ($isError) {
+        $activity = 'api_error';
+    }
+
     $entry = [
         'ts'              => gmdate('c'),
-        'activity'        => $ctx['activity'] ?? null,
+        'activity'        => $activity,
         'method'          => $ctx['method'] ?? '',
         'route'           => $ctx['route'] ?? '',
         'status'          => $status,
@@ -281,11 +305,19 @@ function qdb_api_log_finish(): void
         ),
     ];
 
+    if (!empty($ctx['api_error_code'])) {
+        $entry['error_code']    = (string)$ctx['api_error_code'];
+        $entry['error_message'] = (string)($ctx['api_error_message'] ?? '');
+        $entry['error']         = $entry['error_code'] . ': ' . $entry['error_message'];
+    }
     if (!empty($ctx['exception'])) {
-        $entry['error'] = (string)$ctx['exception'];
+        $entry['error_internal'] = (string)$ctx['exception'];
         if (!empty($ctx['exception_class'])) {
             $entry['error_class'] = (string)$ctx['exception_class'];
         }
+        if (empty($entry['error'])) {
+            $entry['error'] = 'SERVER_ERROR: ' . $entry['error_internal'];
+        }
     }
 
     if (($ctx['user_agent'] ?? '') !== '') {
@@ -563,13 +595,17 @@ function qdb_api_log_resolve_write_path(string $dir): string
  *
  * @return array{date: string, activity: string, entries: list<array<string, mixed>>, files: list<string>, truncated: bool}
  */
-function qdb_api_log_read_entries(string $date, string $activityFilter = '', int $limit = 200): array
+function qdb_api_log_read_entries(string $date, string $activityFilter = '', int $limit = 200, bool $errorsOnly = false): array
 {
     if (!preg_match('/^\d{4}-\d{2}-\d{2}$/', $date)) {
         $date = date('Y-m-d');
     }
     $limit = max(1, min(1000, $limit));
     $activityFilter = strtolower(trim($activityFilter));
+    if ($activityFilter === 'errors') {
+        $errorsOnly     = true;
+        $activityFilter = '';
+    }
     if ($activityFilter !== '' && !in_array($activityFilter, QDB_API_LOG_ACTIVITIES, true)) {
         $activityFilter = '';
     }
@@ -591,6 +627,13 @@ function qdb_api_log_read_entries(string $date, string $activityFilter = '', int
             if ($activityFilter !== '' && ($row['activity'] ?? '') !== $activityFilter) {
                 continue;
             }
+            if ($errorsOnly) {
+                $rowActivity = $row['activity'] ?? '';
+                $rowStatus   = (int)($row['status'] ?? 0);
+                if ($rowActivity !== 'api_error' && $rowStatus < 400 && empty($row['error'])) {
+                    continue;
+                }
+            }
             $entries[] = qdb_api_log_entry_for_ui($row);
         }
     }
@@ -607,13 +650,14 @@ function qdb_api_log_read_entries(string $date, string $activityFilter = '', int
     $entries = qdb_api_log_enrich_entries_usernames($entries);
 
     return [
-        'date'      => $date,
-        'activity'  => $activityFilter,
-        'entries'   => $entries,
-        'files'     => array_map('basename', $files),
-        'truncated' => $truncated,
-        'kinds'     => QDB_API_LOG_ACTIVITIES,
-        'logStatus' => qdb_api_log_status(),
+        'date'       => $date,
+        'activity'   => $activityFilter,
+        'errorsOnly' => $errorsOnly,
+        'entries'    => $entries,
+        'files'      => array_map('basename', $files),
+        'truncated'  => $truncated,
+        'kinds'      => QDB_API_LOG_ACTIVITIES,
+        'logStatus'  => qdb_api_log_status(),
     ];
 }
 
@@ -663,6 +707,8 @@ function qdb_api_log_entry_for_ui(array $row): array
         'changes'         => $changes,
         'summary'         => qdb_api_log_summarize_changes($changes, $targetUsername),
         'error'           => $row['error'] ?? null,
+        'error_code'      => $row['error_code'] ?? null,
+        'error_message'   => $row['error_message'] ?? null,
     ];
 }
 
diff --git a/lib/app_submit_validate.php b/lib/app_submit_validate.php
new file mode 100644
index 0000000..8a4b3e3
--- /dev/null
+++ b/lib/app_submit_validate.php
@@ -0,0 +1,171 @@
+<?php
+
+/**
+ * Validate Android app questionnaire submit payloads before persisting answers.
+ *
+ * @return list<array{questionID: string, code: string, message: string}>
+ */
+function qdb_validate_app_submit_payload(
+    PDO $pdo,
+    string $qnID,
+    array $rawAnswers,
+    array $shortIdMap,
+    array $shortIdToType,
+    array $symptomParentMap,
+    array $optionMap
+): array {
+    $errors = [];
+
+    if ($rawAnswers === []) {
+        $errors[] = [
+            'questionID' => '',
+            'code'       => 'ANSWERS_REQUIRED',
+            'message'    => 'At least one answer is required',
+        ];
+        return $errors;
+    }
+
+    foreach ($rawAnswers as $idx => $a) {
+        if (!is_array($a)) {
+            $errors[] = [
+                'questionID' => '',
+                'code'       => 'INVALID_ANSWER',
+                'message'    => 'Answer at index ' . $idx . ' must be an object',
+            ];
+            continue;
+        }
+        $shortId = trim($a['questionID'] ?? '');
+        if ($shortId === '') {
+            $errors[] = [
+                'questionID' => '',
+                'code'       => 'MISSING_QUESTION_ID',
+                'message'    => 'Each answer must include questionID',
+            ];
+            continue;
+        }
+
+        if (isset($symptomParentMap[$shortId])) {
+            $label = trim((string)($a['answerOptionKey'] ?? $a['freeTextValue'] ?? ''));
+            if ($label === '') {
+                $errors[] = [
+                    'questionID' => $shortId,
+                    'code'       => 'EMPTY_ANSWER',
+                    'message'    => 'Symptom selection is required',
+                ];
+            }
+            continue;
+        }
+
+        if (!isset($shortIdMap[$shortId])) {
+            $errors[] = [
+                'questionID' => $shortId,
+                'code'       => 'UNKNOWN_QUESTION',
+                'message'    => 'Unknown question: ' . $shortId,
+            ];
+            continue;
+        }
+
+        $fullQID = $shortIdMap[$shortId];
+        $type    = $shortIdToType[$shortId] ?? '';
+        $optKey  = $a['answerOptionKey'] ?? null;
+        $free    = isset($a['freeTextValue']) ? trim((string)$a['freeTextValue']) : '';
+        $numeric = $a['numericValue'] ?? null;
+        $hasNumeric = $numeric !== null && $numeric !== '';
+
+        if ($type === 'multi_check_box_question') {
+            if ($optKey !== null && trim((string)$optKey) !== '') {
+                continue;
+            }
+            if ($free !== '') {
+                continue;
+            }
+            $errors[] = [
+                'questionID' => $shortId,
+                'code'       => 'EMPTY_ANSWER',
+                'message'    => 'Select at least one option',
+            ];
+            continue;
+        }
+
+        if ($optKey !== null && trim((string)$optKey) !== '') {
+            $key = (string)$optKey;
+            if (!isset($optionMap[$fullQID][$key])) {
+                $errors[] = [
+                    'questionID' => $shortId,
+                    'code'       => 'INVALID_OPTION',
+                    'message'    => 'Unknown option key: ' . $key,
+                ];
+            }
+            continue;
+        }
+
+        if ($free !== '' || $hasNumeric) {
+            continue;
+        }
+
+        $errors[] = [
+            'questionID' => $shortId,
+            'code'       => 'EMPTY_ANSWER',
+            'message'    => 'Answer is empty',
+        ];
+    }
+
+    if ($errors !== []) {
+        return $errors;
+    }
+
+    require_once __DIR__ . '/app_answers.php';
+    $grouped = qdb_group_app_submit_answers($rawAnswers, $shortIdToType, $symptomParentMap);
+    $answeredShort = [];
+    foreach ($grouped as $row) {
+        $sid = trim($row['questionID'] ?? '');
+        if ($sid !== '') {
+            $answeredShort[$sid] = true;
+        }
+    }
+
+    $qStmt = $pdo->prepare(
+        'SELECT questionID, type, isRequired, configJson FROM question WHERE questionnaireID = :qn'
+    );
+    $qStmt->execute([':qn' => $qnID]);
+    foreach ($qStmt->fetchAll(PDO::FETCH_ASSOC) as $qRow) {
+        if ((int)($qRow['isRequired'] ?? 0) !== 1) {
+            continue;
+        }
+        $fullId = $qRow['questionID'];
+        $parts  = explode('__', $fullId);
+        $short  = end($parts);
+        $type   = $qRow['type'] ?? '';
+
+        if ($type === 'glass_scale_question') {
+            $cfg      = json_decode($qRow['configJson'] ?? '{}', true) ?: [];
+            $symptoms = $cfg['symptoms'] ?? [];
+            if (is_array($symptoms) && $symptoms !== []) {
+                foreach ($symptoms as $symptomKey) {
+                    $sk = trim((string)$symptomKey);
+                    if ($sk === '') {
+                        continue;
+                    }
+                    if (empty($answeredShort[$sk])) {
+                        $errors[] = [
+                            'questionID' => $sk,
+                            'code'       => 'MISSING_ANSWER',
+                            'message'    => 'Required symptom is not answered',
+                        ];
+                    }
+                }
+                continue;
+            }
+        }
+
+        if (empty($answeredShort[$short])) {
+            $errors[] = [
+                'questionID' => $short,
+                'code'       => 'MISSING_ANSWER',
+                'message'    => 'Required question is not answered',
+            ];
+        }
+    }
+
+    return $errors;
+}
diff --git a/lib/errors.php b/lib/errors.php
new file mode 100644
index 0000000..81f03fd
--- /dev/null
+++ b/lib/errors.php
@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Map internal exceptions to safe, identifiable API error messages (details go to error_log).
+ */
+function qdb_public_error_message(Throwable $e, string $contextLabel = 'Operation'): string
+{
+    $msg = $e->getMessage();
+
+    if (str_contains($msg, 'Could not acquire lock') || str_contains($msg, 'Could not open lock')) {
+        return 'Database is busy. Please try again in a moment.';
+    }
+    if (str_contains($msg, 'decrypt') || str_contains($msg, 'QDB_MASTER_KEY')) {
+        return 'Database configuration error. Contact the server administrator.';
+    }
+    if (str_contains($msg, 'Could not read') || str_contains($msg, 'Could not write')
+        || str_contains($msg, 'Could not save')) {
+        return 'Database storage error. Contact the server administrator.';
+    }
+    if ($e instanceof PDOException) {
+        return $contextLabel . ' failed due to a database error.';
+    }
+
+    return $contextLabel . ' failed. If this persists, contact support.';
+}
diff --git a/lib/response.php b/lib/response.php
index 3476562..c1988ff 100644
--- a/lib/response.php
+++ b/lib/response.php
@@ -9,10 +9,20 @@ function json_success(mixed $data, int $status = 200): never {
     exit;
 }
 
-function json_error(string $code, string $message, int $status = 400): never {
+/**
+ * @param mixed $details Optional structured payload (e.g. validation error list).
+ */
+function json_error(string $code, string $message, int $status = 400, mixed $details = null): never {
+    if (function_exists('qdb_api_log_note_api_error')) {
+        qdb_api_log_note_api_error($code, $message, $status);
+    }
     http_response_code($status);
     header('Content-Type: application/json; charset=UTF-8');
-    echo json_encode(["ok" => false, "error" => ["code" => $code, "message" => $message]]);
+    $error = ["code" => $code, "message" => $message];
+    if ($details !== null) {
+        $error['details'] = $details;
+    }
+    echo json_encode(["ok" => false, "error" => $error]);
     exit;
 }
 
diff --git a/website/js/api.js b/website/js/api.js
index 93fa0c9..38be401 100644
--- a/website/js/api.js
+++ b/website/js/api.js
@@ -27,14 +27,17 @@ export function redirectToLogin() {
 /** @returns {boolean} true if the response means the session is no longer valid */
 function isSessionInvalidResponse(res, json) {
     if (res.status === 401) return true;
-    const errMsg = json.error?.message || json.error || '';
-    return res.status === 403 && (
-        errMsg === 'Invalid or expired token'
-        || errMsg === 'Missing Bearer token'
-        || errMsg === 'Password change required before access'
-        || json.error?.code === 'UNAUTHORIZED'
-        || json.error?.code === 'PASSWORD_CHANGE_REQUIRED'
-    );
+    const errMsg = json.error?.message || (typeof json.error === 'string' ? json.error : '');
+    const errCode = json.error?.code || '';
+    return res.status === 401
+        || res.status === 403 && (
+            errCode === 'UNAUTHORIZED'
+            || errCode === 'PASSWORD_CHANGE_REQUIRED'
+            || errCode === 'FORBIDDEN'
+            || errMsg === 'Invalid or expired token'
+            || errMsg === 'Missing Bearer token'
+            || errMsg === 'Password change required before access'
+        );
 }
 
 /**
@@ -125,7 +128,9 @@ function unwrap(json) {
             const d = json.data || {};
             return { success: true, ...(Array.isArray(d) ? { items: d } : d) };
         }
-        return { success: false, error: json.error?.message || 'Unknown error' };
+        const err = json.error;
+        const msg = typeof err === 'string' ? err : (err?.message || 'Unknown error');
+        return { success: false, error: msg, errorCode: typeof err === 'object' ? err?.code : null };
     }
     return json;
 }
diff --git a/website/js/pages/dev.js b/website/js/pages/dev.js
index a2625d2..be67724 100644
--- a/website/js/pages/dev.js
+++ b/website/js/pages/dev.js
@@ -7,6 +7,7 @@ const ACTIVITY_LABELS = {
     app_change: 'App change',
     web_change: 'Website change',
     web_export: 'Website export',
+    api_error: 'API error',
 };
 
 export function devPage() {
@@ -75,6 +76,7 @@ export function devPage() {
                     <option value="app_change">App change</option>
                     <option value="web_change">Website change</option>
                     <option value="web_export">Website export</option>
+                    <option value="errors">Errors only (4xx/5xx)</option>
                 </select>
                 <button type="button" class="btn btn-sm" id="activityLogRefresh">Refresh</button>
             </div>
@@ -324,7 +326,9 @@ async function loadActivityLog() {
             const label = ACTIVITY_LABELS[kind] || kind || '—';
             const badgeClass = kind ? `badge-activity-${kind}` : '';
             const time = formatActivityTime(e.ts);
-            const status = e.status != null ? String(e.status) : '—';
+            const statusNum = e.status != null ? Number(e.status) : null;
+            const status = statusNum != null ? String(statusNum) : '—';
+            const statusClass = statusNum != null && statusNum >= 400 ? 'error-text' : '';
             const dur = e.duration_ms != null ? `${e.duration_ms} ms` : '—';
             const subject = formatActivitySubject(e);
             const performedBy = formatActivityPerformer(e);
@@ -339,7 +343,7 @@ async function loadActivityLog() {
                 <td><code>${escHtml(e.method || '')}</code></td>
                 <td><code>${escHtml(e.route || '')}</code></td>
                 <td class="activity-log-ip"><code>${escHtml(ip)}</code></td>
-                <td>${escHtml(status)}</td>
+                <td class="${statusClass}">${escHtml(status)}</td>
                 <td>${escHtml(dur)}</td>
                 <td class="activity-log-changes-cell">${renderActivityChanges(e.changes)}</td>
                 <td class="activity-log-actor">${performedBy}${errPart}</td>