docs(api): complete backend documentation with all missing endpoints

Complete API documentation audit covering all backend endpoints.

Auth Endpoints:
- Fixed endpoint paths from /api/users/me/* to /api/auth/*
- Added update_email.md and update_username.md

Device Management:
- Device shelf operations (add, get, remove, clear)
- Device CRUD operations (update, delete)
- Registration management (pending, approve, reject)

Books Operations:
- Bulk delete and bulk update with normalization
- Download endpoint with format-specific headers

Conflict Resolution:
- Complete conflict management (list, get, resolve, delete)
- Bulk operations (bulk resolve, bulk dismiss, dismiss all)

Sync Protocols:
- KOReader: progress, metadata, library, bookmarks
- Kobo: markup, bookmarks, analytics, initialization, server sync

Scanner:
- Enhanced docs with manga/comic support
- Added 148-line comprehensive overview
- All formats documented (ebooks, comics, manga)

WebSocket:
- Comprehensive real-time sync API
- Messages, topics, authentication, examples

Documentation:
- Updated api-reference.md with all 122+ endpoints
- Updated index.md with new categories

Total: 36 new files, 6 modified files, 100% coverage

docs/developer/api/scanner/get_watch_mode_status.md

@@ -0,0 +1,66 @@
+# Get Watch Mode Status
+
+Get the watch mode status for a library.
+
+**Endpoint**: `GET /api/scanner/watch/status`
+**Auth**: Required (Admin only)
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| library_id | string (UUID) | Yes | Library UUID to check |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token (must have admin role) |
+
+### Example Request
+
+```http
+GET /api/scanner/watch/status?library_id=550e8400-e29b-41d4-a716-446655440000
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "library_id": "uuid",
+  "status": "watching",
+  "started_at": "2026-02-08T10:00:00Z",
+  "watched_folders": [
+    "/path/to/library/folder1",
+    "/path/to/library/folder2"
+  ],
+  "stats": {
+    "files_detected": 15,
+    "files_processed": 12,
+    "files_failed": 1,
+    "last_activity": "2026-02-08T11:30:00Z"
+  }
+}
+```
+
+## Status Values
+
+| Status | Description |
+|--------|-------------|
+| watching | Watch mode is active |
+| stopped | Watch mode is not active |
+| error | Watch mode encountered an error |
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Missing library_id parameter |
+| 401 | Invalid or expired token |
+| 403 | User does not have admin privileges |
+| 404 | Library not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->