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/bulk_update_books.md

@@ -0,0 +1,71 @@
+# Bulk Update Books
+
+Update multiple media items at once.
+
+**Endpoint**: `POST /api/books/bulk-update`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| media_item_ids | array of UUID | Yes | Array of media item UUIDs to update |
+| updates | object | Yes | Fields to update (see below) |
+
+### Update Fields
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| tags | array of strings | No | New tags (will replace existing) |
+| contributors | array of strings | No | New contributors (will replace existing) |
+| add_tags | array of strings | No | Tags to add to existing |
+| remove_tags | array of strings | No | Tags to remove from existing |
+
+### Example Request
+
+```json
+{
+  "media_item_ids": [
+    "550e8400-e29b-41d4-a716-446655440001",
+    "660e8400-e29b-41d4-a716-446655440002"
+  ],
+  "updates": {
+    "add_tags": ["science-fiction", "award-winner"],
+    "contributors": ["Publisher Name"]
+  }
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Books updated successfully",
+  "updated_count": 2,
+  "failed_count": 0,
+  "normalized_tags": ["Science-Fiction", "Award-Winner"],
+  "normalized_contributors": ["Publisher Name"]
+}
+```
+
+## Tag and Contributor Normalization
+
+The backend automatically normalizes tags and contributors:
+
+- **Tags**: Titlecased, punctuation preserved, deduplicated
+- **Contributors**: Original casing preserved, punctuation preserved
+- **Search fields**: Lowercase, no punctuation, stored in `tags_search` and `contributors_search`
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data |
+| 401 | Invalid or expired token |
+| 403 | User does not have permission |
+| 404 | One or more media items not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->