added categoryKey to questionnaire schema and updated related functions for questionnaire handling; enhanced API to fetch client answers

docs/android-questionnaire-api.md

@@ -149,6 +149,7 @@ Fields:
 - `name`: display/admin name.
 - `showPoints`: whether the app may show the points result.
 - `condition`: JSON condition object. Empty conditions are returned as `{}`.
+- `categoryKey` (optional): grouping key for the opening screen; labels come from app UI strings (e.g. `questionnaire_group_health_interview`).
 
 Only questionnaires with `state = "active"` are returned, ordered by server `orderIndex`.
 
@@ -207,6 +208,57 @@ Question fields:
 
 Important ID rule: the fetch response returns short question IDs such as `q1`. The server resolves these back to full database IDs during upload, so the Android app should upload the same short IDs it received.
 
+## Download Client Answers (restore on device)
+
+`GET /api/app_questionnaires?clientCode=<code>&answers=1`
+
+Allowed roles: `coach`, `supervisor`, `admin` (coach must be assigned to the client).
+
+Headers:
+
+```http
+Authorization: Bearer <token>
+```
+
+Success (encrypted envelope, same as `?clients=1`):
+
+```json
+{
+  "ok": true,
+  "data": {
+    "encrypted": true,
+    "payload": { "...": "..." }
+  }
+}
+```
+
+Decrypted `data`:
+
+```json
+{
+  "clientCode": "K008",
+  "questionnaires": [
+    {
+      "questionnaireID": "questionnaire_2_rhs",
+      "completedAt": 1710000000,
+      "sumPoints": 12,
+      "answers": [
+        { "questionID": "q1", "answerOptionKey": "consent_signed" },
+        { "questionID": "languages_spoken", "answerOptionKey": "language_arabic" },
+        { "questionID": "languages_spoken", "answerOptionKey": "language_english" }
+      ]
+    }
+  ]
+}
+```
+
+Notes:
+
+- Returns all `completed_questionnaire` rows for the client.
+- `answers` mirrors the POST submit shape (short `questionID`, glass-scale symptoms as separate entries, multi-checkbox as multiple rows with the same `questionID`).
+- On the server, multi-checkbox answers are stored in one `client_answer` row (`freeTextValue` JSON array); export expands them for the app importer.
+- Use on **client load** to restore answers after upload or on a fresh install.
+
 ## Fetch Assigned Clients
 
 `GET /api/app_questionnaires?clients=1`