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/api/src/openapi.rs

@@ -46,6 +46,13 @@ use utoipa::OpenApi;
         crate::settings::clear_cache,
         crate::settings::get_cache_stats,
         crate::settings::get_thumbnail_stats,
+        crate::metadata::search_metadata,
+        crate::metadata::create_metadata_match,
+        crate::metadata::approve_metadata,
+        crate::metadata::reject_metadata,
+        crate::metadata::get_metadata_links,
+        crate::metadata::get_missing_books,
+        crate::metadata::delete_metadata_link,
     ),
     components(
         schemas(
@@ -94,6 +101,18 @@ use utoipa::OpenApi;
             crate::stats::LibraryStats,
             crate::stats::TopSeries,
             crate::stats::MonthlyAdditions,
+            crate::metadata::ApproveRequest,
+            crate::metadata::ApproveResponse,
+            crate::metadata::SyncReport,
+            crate::metadata::SeriesSyncReport,
+            crate::metadata::BookSyncReport,
+            crate::metadata::FieldChange,
+            crate::metadata::MetadataSearchRequest,
+            crate::metadata::SeriesCandidateDto,
+            crate::metadata::MetadataMatchRequest,
+            crate::metadata::ExternalMetadataLinkDto,
+            crate::metadata::MissingBooksDto,
+            crate::metadata::MissingBookItem,
             ErrorResponse,
         )
     ),