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/kobo/initialization.md

@@ -0,0 +1,53 @@
+# Kobo Initialization
+
+Initialize Kobo device sync.
+
+**Endpoint**: `GET /api/sync/kobo/v1/initialization`
+**Auth**: Required (Device authentication)
+
+## Device Authentication
+
+This endpoint requires device authentication (not user JWT). Kobo devices authenticate using their device credentials and access tokens.
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| X-Device-ID | string | Yes | Device UUID |
+| X-Device-Key | string | Yes | Device authentication key |
+| X-Kobo-UserKey | string | No | Kobo user key (if available) |
+
+### Example Request
+
+```http
+GET /api/sync/kobo/v1/initialization
+X-Device-ID: 550e8400-e29b-41d4-a716-446655440000
+X-Device-Key: device-auth-key
+```
+
+## Response (200 OK)
+
+```json
+{
+  "result": "ok",
+  "resources": {
+    "image_host": "https://cdn.kobo.com/...",
+    "image_url_quality_template": "{ImageId}/h270/{ImageId}_landscape_cover.jpg"
+  }
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Device authentication failed |
+| 404 | Device not found |
+
+## Notes
+
+This endpoint is called by Kobo devices during initial setup and connection. It returns configuration data required for proper Kobo integration.
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->