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/start_scanner.md

@@ -0,0 +1,54 @@
+# Start Scanner
+
+Start the automated background scanner for a library. Supports ebook, manga, and comic libraries.
+
+**Endpoint**: `POST /api/scanner/start`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| library_id | string (UUID) | Yes | Library UUID to scan (supports ebooks, manga, and comics) |
+| interval_seconds | integer | No | Scan interval in seconds (default: 3600, min: 300) |
+
+## Supported Library Types
+
+- **Ebooks:** Processes .epub, .pdf, .mobi, and other ebook formats
+- **Comics:** Processes comic archives (.cbz, .cbr, .cb7, .cbt) with metadata extraction
+- **Manga:** Processes manga archives and image files (.cbz, .cbr, .png, .jpg, etc.)
+
+### Example Request
+
+```json
+{
+  "library_id": "550e8400-e29b-41d4-a716-446655440000",
+  "interval_seconds": 3600
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Scanner started successfully",
+  "library_id": "uuid",
+  "interval_seconds": 3600,
+  "status": "running"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data or interval too low |
+| 401 | Invalid or expired token |
+| 403 | User does not have admin privileges |
+| 404 | Library not found |
+| 409 | Scanner already running for this library |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->