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/approve/route.ts

@@ -0,0 +1,17 @@
+import { NextRequest, NextResponse } from "next/server";
+import { apiFetch } from "@/lib/api";
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { id, ...rest } = body;
+    const data = await apiFetch<{ status: string; books_synced: number }>(`/metadata/approve/${id}`, {
+      method: "POST",
+      body: JSON.stringify(rest),
+    });
+    return NextResponse.json(data);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Failed to approve metadata";
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}