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/0028_add_external_metadata.sql

@@ -0,0 +1,41 @@
+CREATE TABLE external_metadata_links (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    library_id UUID NOT NULL REFERENCES libraries(id) ON DELETE CASCADE,
+    series_name TEXT NOT NULL,
+    provider TEXT NOT NULL,
+    external_id TEXT NOT NULL,
+    external_url TEXT,
+    status TEXT NOT NULL DEFAULT 'pending',
+    confidence REAL,
+    metadata_json JSONB NOT NULL DEFAULT '{}',
+    total_volumes_external INTEGER,
+    matched_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    approved_at TIMESTAMPTZ,
+    synced_at TIMESTAMPTZ,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    UNIQUE (library_id, series_name, provider)
+);
+
+CREATE TABLE external_book_metadata (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    link_id UUID NOT NULL REFERENCES external_metadata_links(id) ON DELETE CASCADE,
+    book_id UUID REFERENCES books(id) ON DELETE SET NULL,
+    external_book_id TEXT,
+    volume_number INTEGER,
+    title TEXT,
+    authors TEXT[] NOT NULL DEFAULT '{}',
+    isbn TEXT,
+    summary TEXT,
+    cover_url TEXT,
+    page_count INTEGER,
+    language TEXT,
+    publish_date TEXT,
+    metadata_json JSONB NOT NULL DEFAULT '{}',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_eml_library_series ON external_metadata_links(library_id, series_name);
+CREATE INDEX idx_eml_status ON external_metadata_links(status);
+CREATE INDEX idx_ebm_link_id ON external_book_metadata(link_id);
+CREATE INDEX idx_ebm_book_id ON external_book_metadata(book_id);

infra/migrations/0029_add_library_metadata_provider.sql

@@ -0,0 +1 @@
+ALTER TABLE libraries ADD COLUMN metadata_provider TEXT;

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 '{}';

infra/migrations/0031_add_total_volumes_to_series_metadata.sql

@@ -0,0 +1 @@
+ALTER TABLE series_metadata ADD COLUMN total_volumes INTEGER;

infra/migrations/0032_add_book_metadata_fields.sql

@@ -0,0 +1,4 @@
+-- Add metadata fields to books table for external sync
+ALTER TABLE books ADD COLUMN summary TEXT;
+ALTER TABLE books ADD COLUMN isbn TEXT;
+ALTER TABLE books ADD COLUMN publish_date TEXT;