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/Cargo.toml

@@ -19,6 +19,7 @@ stripstream-core = { path = "../../crates/core" }
 parsers = { path = "../../crates/parsers" }
 rand.workspace = true
 tokio-stream = "0.1"
+regex = "1"
 reqwest.workspace = true
 serde.workspace = true
 serde_json.workspace = true
@@ -33,3 +34,4 @@ uuid.workspace = true
 utoipa.workspace = true
 utoipa-swagger-ui = { workspace = true, features = ["axum"] }
 webp.workspace = true
+scraper.workspace = true