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/books/download_book.md

@@ -0,0 +1,56 @@
+# Download Book
+
+Download a book file.
+
+**Endpoint**: `GET /api/books/:uuid/download`
+**Auth**: Not required (public endpoint)
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| uuid | string (UUID) | Yes | Media item UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | No* | Bearer token (required if media item is not public) |
+
+\* If the media item is in a non-public library, authentication is required.
+
+### Example Request
+
+```http
+GET /api/books/550e8400-e29b-41d4-a716-446655440000/download
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+Binary file download with appropriate Content-Type header.
+
+**Possible Content-Types:**
+- application/epub+zip
+- application/pdf
+- application/x-mobipocket-ebook
+- application/octet-stream
+
+**Headers:**
+```
+Content-Type: application/epub+zip
+Content-Disposition: attachment; filename="book-title.epub"
+Content-Length: 1234567
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Authentication required for non-public content |
+| 404 | Media item not found |
+| 404 | Book file not found on disk |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->