feat: add metadata provider filter to series page

- Add `metadata_provider` query param to series API endpoints (linked/unlinked/specific provider)
- Return `metadata_provider` field in series response
- Add metadata filter dropdown on series page with all provider options
- Show small provider icon badge on linked series cards
- LiveSearchForm now wraps filters on two rows when needed

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

apps/backoffice/lib/i18n/en.ts

@@ -550,6 +550,12 @@ const en: Record<TranslationKey, string> = {
   // Series filters
   "seriesFilters.all": "All",
   "seriesFilters.missingBooks": "Missing books",
+
+  // Metadata filter
+  "series.metadata": "Metadata",
+  "series.metadataAll": "All",
+  "series.metadataLinked": "Linked",
+  "series.metadataUnlinked": "Not linked",
 };
 
 export default en;