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/app/components/LiveSearchForm.tsx

@@ -18,6 +18,10 @@ const FILTER_ICONS: Record<string, string> = {
   metadata_provider: "M7 7h.01M7 3h5c.512 0 1.024.195 1.414.586l7 7a2 2 0 010 2.828l-7 7a2 2 0 01-2.828 0l-7-7A1.994 1.994 0 013 12V7a4 4 0 014-4z",
   // Sort - arrows up/down
   sort: "M3 4h13M3 8h9m-9 4h6m4 0l4-4m0 0l4 4m-4-4v12",
+  // Format - document/file
+  format: "M7 21h10a2 2 0 002-2V9.414a1 1 0 00-.293-.707l-5.414-5.414A1 1 0 0012.586 3H7a2 2 0 00-2 2v14a2 2 0 002 2z",
+  // Metadata - link/chain
+  metadata: "M13.828 10.172a4 4 0 00-5.656 0l-4 4a4 4 0 105.656 5.656l1.102-1.101m-.758-4.899a4 4 0 005.656 0l4-4a4 4 0 00-5.656-5.656l-1.1 1.1",
 };
 
 interface FieldDef {