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/app/api/metadata/links/route.ts

@@ -0,0 +1,35 @@
+import { NextRequest, NextResponse } from "next/server";
+import { apiFetch, ExternalMetadataLinkDto } from "@/lib/api";
+
+export async function GET(request: NextRequest) {
+  try {
+    const { searchParams } = new URL(request.url);
+    const libraryId = searchParams.get("library_id") || "";
+    const seriesName = searchParams.get("series_name") || "";
+    const params = new URLSearchParams();
+    if (libraryId) params.set("library_id", libraryId);
+    if (seriesName) params.set("series_name", seriesName);
+    const data = await apiFetch<ExternalMetadataLinkDto[]>(`/metadata/links?${params.toString()}`);
+    return NextResponse.json(data);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Failed to fetch metadata links";
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}
+
+export async function DELETE(request: NextRequest) {
+  try {
+    const { searchParams } = new URL(request.url);
+    const id = searchParams.get("id");
+    if (!id) {
+      return NextResponse.json({ error: "id is required" }, { status: 400 });
+    }
+    const data = await apiFetch<{ deleted: boolean }>(`/metadata/links/${id}`, {
+      method: "DELETE",
+    });
+    return NextResponse.json(data);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Failed to delete metadata link";
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}