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/overview.md

@@ -0,0 +1,148 @@
+# Library Scanner Overview
+
+The Bookhoard scanner provides comprehensive library management for ebooks, comics, and manga with both manual and automated scanning capabilities.
+
+## Scanner Types
+
+### Manual Scanning
+- **One-time scan**: On-demand scanning of library folders
+- **Progress tracking**: Real-time status updates with file-by-file progress
+- **Error reporting**: Detailed logs of failed files with error messages
+
+### Automated Scanner
+- **Periodic scanning**: Configurable interval-based background scanning
+- **Automatic discovery**: Detects new files without manual intervention
+- **Resource-efficient**: Minimizes system impact with smart scheduling
+
+### Watch Mode
+- **Real-time monitoring**: Instant detection of file system changes
+- **Event-driven**: Processes files immediately upon addition/modification
+- **Platform support**: Works with inotify (Linux), FSEvents (macOS), and ReadDirectoryChangesW (Windows)
+
+## Supported Formats
+
+### Ebooks
+| Format | Extensions |
+|--------|------------|
+| EPUB | `.epub` |
+| PDF | `.pdf` |
+| Kindle | `.mobi`, `.azw`, `.azw3` |
+| Text | `.txt`, `.rtf` |
+| Document | `.doc`, `.docx` |
+| Other | `.lit`, `.fb2`, `.pdb` |
+
+### Comics
+| Format | Extensions | Archive Type |
+|--------|------------|--------------|
+| Comic Book ZIP | `.cbz` | ZIP |
+| Comic Book RAR | `.cbr` | RAR |
+| Comic Book 7z | `.cb7` | 7-Zip |
+| Comic Book TAR | `.cbt` | TAR |
+| PDF Comics | `.pdf` | PDF |
+
+### Manga
+| Format | Extensions | Notes |
+|--------|------------|-------|
+| Comic Archives | `.cbz`, `.cbr` | Same as comics |
+| Image Files | `.png`, `.jpg`, `.jpeg`, `.gif`, `.bmp`, `.webp` | Individual pages |
+
+## Metadata Extraction
+
+### Comic Archives (.cbz, .cbr, .cb7, .cbt)
+The scanner automatically extracts metadata from comic archives:
+
+**ComicInfo.xml Support:**
+- Series title
+- Issue number
+- Publisher
+- Writer, artist, inker, colorist
+- Year, month
+- Genre, tags
+- Cover image extraction
+
+**Fallback Metadata:**
+- Filename parsing
+- Archive structure analysis
+- Page count detection
+
+### Manga Processing
+- **Archive-based**: Processes .cbz/.cbr files like comics
+- **Image-based**: Handles directories of sequential images
+- **Chapter detection**: Identifies chapter/volume numbers from filenames
+- **Series grouping**: Groups images into logical manga volumes
+
+## Scanner Features
+
+### Smart Deduplication
+- SHA256 hash calculation for all files
+- Automatic duplicate detection and skipping
+- Efficient incremental updates
+
+### Library Type Awareness
+- Format filtering based on library type
+- Type-specific metadata extraction
+- Appropriate thumbnail generation
+
+### Error Handling
+- Continues on individual file errors
+- Detailed error reporting in scan status
+- Failed file tracking for retry
+
+### Progress Tracking
+- Total files vs. processed files
+- Percentage completion
+- Added, updated, and failed file counts
+- Per-file error messages
+
+## Performance Considerations
+
+### Large Libraries
+- **Scanning speed**: Processes hundreds of files per second
+- **Memory usage**: Streaming metadata extraction
+- **Database efficiency**: Batch inserts and updates
+
+### Resource Limits
+- **Configurable intervals**: Prevent excessive scanning
+- **Rate limiting**: Watch mode debounce settings
+- **Admin controls**: Start/stop operations as needed
+
+## Usage Examples
+
+### Create and Scan a Comic Library
+```json
+POST /api/libraries
+{
+  "name": "My Comic Collection",
+  "library_type_id": "comics-type-uuid",
+  "description": "Marvel and DC comics"
+}
+
+POST /api/scanner/scan
+{
+  "library_id": "library-uuid",
+  "recursive": true
+}
+```
+
+### Enable Watch Mode for Manga
+```json
+POST /api/scanner/watch/start
+{
+  "library_id": "manga-library-uuid"
+}
+```
+
+### Check Scan Progress
+```http
+GET /api/scanner/status/550e8400-e29b-41d4-a716-446655440000
+```
+
+## Related Endpoints
+
+- [Scan Ebooks](scan_ebooks.md) - Manual one-time scan
+- [Start Scanner](start_scanner.md) - Automated periodic scanning
+- [Stop Scanner](stop_scanner.md) - Stop automated scanner
+- [Get Scan Status](get_scan_status.md) - Check scan progress
+- [Start Watch Mode](start_watch_mode.md) - Enable real-time monitoring
+- [Stop Watch Mode](stop_watch_mode.md) - Disable watch mode
+- [Get Watch Mode Status](get_watch_mode_status.md) - Check watch status