feat: add format and metadata filters to books page

Add two new filters to the books listing page:
- Format filter (CBZ/CBR/PDF/EPUB) using existing API support
- Metadata linked/unlinked filter with new API support via
  LEFT JOIN on external_metadata_links (using DISTINCT ON CTE
  matching the series endpoint pattern)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/lib/i18n/en.ts

@@ -100,6 +100,8 @@ const en: Record<TranslationKey, string> = {
   "books.noResults": "No books found for \"{{query}}\"",
   "books.noBooks": "No books available",
   "books.coverOf": "Cover of {{name}}",
+  "books.format": "Format",
+  "books.allFormats": "All formats",
 
   // Series page
   "series.title": "Series",