feat: include series_count and thumbnail_book_ids in libraries API response

Eliminates N+1 sequential fetchSeries calls on the libraries page by
returning series count and up to 5 thumbnail book IDs (one per series)
directly from GET /libraries.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/backoffice/lib/api.ts

@@ -12,6 +12,8 @@ export type LibraryDto = {
   fallback_metadata_provider: string | null;
   metadata_refresh_mode: string;
   next_metadata_refresh_at: string | null;
+  series_count: number;
+  thumbnail_book_ids: string[];
 };
 
 export type IndexJobDto = {