feat: add external metadata sync system with multiple providers

Add a complete metadata synchronization system allowing users to search
and sync series/book metadata from external providers (Google Books,
Open Library, ComicVine, AniList, Bédéthèque). Each library can use a
different provider. Matching requires manual approval with detailed sync
reports showing what was updated or skipped (locked fields protection).

Key changes:
- DB migrations: external_metadata_links, external_book_metadata tables,
  library metadata_provider column, locked_fields, total_volumes, book
  metadata fields (summary, isbn, publish_date)
- Rust API: MetadataProvider trait + 5 provider implementations,
  7 metadata endpoints (search, match, approve, reject, links, missing,
  delete), sync report system, provider language preference support
- Backoffice: MetadataSearchModal, ProviderIcon, SafeHtml components,
  settings UI for provider/language config, enriched book detail page,
  edit forms with locked fields support, API proxy routes
- OpenAPI/Swagger documentation for all new endpoints and schemas

Closes #3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/package.json

@@ -11,13 +11,15 @@
     "next": "^16.1.6",
     "next-themes": "^0.4.6",
     "react": "19.0.0",
-    "react-dom": "19.0.0"
+    "react-dom": "19.0.0",
+    "sanitize-html": "^2.17.1"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4.2.1",
     "@types/node": "22.13.14",
     "@types/react": "19.0.12",
     "@types/react-dom": "19.0.5",
+    "@types/sanitize-html": "^2.16.1",
     "autoprefixer": "^10.4.27",
     "postcss": "^8.5.8",
     "tailwindcss": "^4.2.1",