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/index.md

@@ -27,10 +27,19 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
 
 **[Developer Documentation Portal](developer/development.md)** - Technical documentation & API reference
 
-- **API Documentation**
-  - [Complete API Reference](developer/api-reference.md) - Monolithic REST API reference (1,300+ lines)
-   - [Split Endpoint Docs](developer/api/api-reference.md) - Individual endpoints with interactive API Explorer
-  - [Collections API](developer/collections-api.md) - Collections management API
+ - **API Documentation**
+   - [Complete API Reference](developer/api-reference.md) - Monolithic REST API reference (1,300+ lines)
+    - [Split Endpoint Docs](developer/api/api-reference.md) - Individual endpoints with interactive API Explorer
+   - [Collections API](developer/collections-api.md) - Collections management API
+   - [Admin API](developer/api/admin/) - User management (admin operations)
+   - [Books API](developer/api/books/) - Bulk book operations and downloads
+   - [Devices API](developer/api/devices/) - Device sync and shelf management
+   - [Conflicts API](developer/api/conflicts/) - Sync conflict resolution endpoints
+   - [Queue API](developer/api/queue/) - Sync queue management endpoints
+   - [Scanner API](developer/api/scanner/) - Library scanning and automated watch mode (admin)
+   - [KOReader API](developer/api/koreader/) - KOReader sync protocol endpoints
+   - [Kobo API](developer/api/kobo/) - Kobo sync protocol endpoints
+   - [WebSocket API](developer/api/websocket/) - Real-time sync events
 
 - **Protocol Specifications**
   - [Kobo Sync Protocol](developer/api/sync/kobo-protocol.md) - Kobo device sync
@@ -133,5 +142,5 @@ Keep [PROJECT_GUIDELINES.md](PROJECT_GUIDELINES.md) in mind for documentation st
 
 ---
 
-**Last Updated**: 2026-02-02
+**Last Updated**: 2026-02-08
 **Bookhoard Version**: 1.0