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/sync_from_server.md

@@ -0,0 +1,67 @@
+# Sync From Server
+
+Push content and metadata to Kobo device.
+
+**Endpoint**: `POST /api/sync/kobo/sync-from-server`
+**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 Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| book_ids | array of string | No | Array of ContentIDs to sync |
+| full_sync | boolean | No | Whether to perform full sync (default: false) |
+
+### Example Request
+
+```json
+{
+  "book_ids": [
+    "content-id-1",
+    "content-id-2"
+  ],
+  "full_sync": false
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "books": [
+    {
+      "ContentID": "content-id-1",
+      "Title": "Book Title",
+      "Author": "Author Name",
+      "ISBN": "978-0123456789",
+      "PublicationDate": "2026-02-08",
+      "FileSize": 1234567,
+      "CoverImageId": "cover-id-1",
+      "Metadata": {
+        "language": "en",
+        "publisher": "Publisher Name"
+      }
+    }
+  ],
+  "total": 2
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Device authentication failed |
+| 404 | Device not found |
+
+## Notes
+
+This endpoint pushes library content from the server to the Kobo device. It's called during sync operations to update the device's library catalog.
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->