Implement bulk export of client answers for coaches and enhance database handling in db_init.php

lib/app_answers.php

@@ -255,3 +255,30 @@ function qdb_export_app_client_answers_bundle(PDO $pdo, string $clientCode): arr
     }
     return $items;
 }
+
+/**
+ * Bulk export: all coach-assigned clients with full answer bundles (caller applies RBAC).
+ *
+ * @return list<array{clientCode: string, questionnaires: list<array<string, mixed>>}>
+ */
+function qdb_export_app_bulk_answers_for_coach(PDO $pdo, array $tokenRec): array {
+    if (($tokenRec['role'] ?? '') !== 'coach') {
+        return [];
+    }
+    $coachID = trim((string)($tokenRec['entityID'] ?? ''));
+    if ($coachID === '') {
+        return [];
+    }
+    $stmt = $pdo->prepare(
+        'SELECT clientCode FROM client WHERE coachID = :cid ORDER BY clientCode'
+    );
+    $stmt->execute([':cid' => $coachID]);
+    $out = [];
+    foreach ($stmt->fetchAll(PDO::FETCH_COLUMN) as $clientCode) {
+        $out[] = [
+            'clientCode'     => $clientCode,
+            'questionnaires' => qdb_export_app_client_answers_bundle($pdo, (string)$clientCode),
+        ];
+    }
+    return $out;
+}