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>

infra/migrations/0030_add_locked_fields.sql

@@ -0,0 +1,6 @@
+-- Add locked_fields to series_metadata and books so that manually edited
+-- fields can be protected from external-metadata sync overwrites.
+-- The JSON object maps field names to true, e.g. {"authors": true, "description": true}.
+
+ALTER TABLE series_metadata ADD COLUMN locked_fields JSONB NOT NULL DEFAULT '{}';
+ALTER TABLE books ADD COLUMN locked_fields JSONB NOT NULL DEFAULT '{}';