feat: add batch metadata jobs, series filters, and translate backoffice to French

- Add metadata_batch job type with background processing via tokio::spawn
- Auto-apply metadata only when single result at 100% confidence
- Support primary + fallback provider per library, "none" to opt out
- Add batch report/results API endpoints and job detail UI
- Add series_status and has_missing filters to both series listing pages
- Add GET /series/statuses endpoint for dynamic filter options
- Normalize series_metadata status values (migration 0036)
- Hide ComicVine provider tab when no API key configured
- Translate entire backoffice UI from English to French

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

apps/backoffice/app/components/BookCard.tsx

@@ -71,7 +71,7 @@ export function BookCard({ book, readingStatus }: BookCardProps) {
       <div className="relative">
         <BookImage
           src={coverUrl}
-          alt={`Cover of ${book.title}`}
+          alt={`Couverture de ${book.title}`}
         />
         {overlay && (
           <span className={`absolute bottom-2 left-2 px-2 py-0.5 rounded-full text-[10px] font-bold tracking-wide ${overlay.className}`}>