feat: add rescan job type and improve full rebuild UX

Add "Deep rescan" job type that clears directory mtimes to force
re-walking all directories, discovering newly supported formats (e.g.
EPUB) without deleting existing data or metadata.

Also improve full rebuild button: red destructive styling instead of
warning, and FR description explicitly mentions metadata/reading status
loss. Rename FR rebuild label to "Mise à jour".

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

apps/backoffice/app/jobs/[id]/page.tsx

@@ -102,6 +102,11 @@ export default async function JobDetailPage({ params }: JobDetailPageProps) {
       description: t("jobType.full_rebuildDesc"),
       isThumbnailOnly: false,
     },
+    rescan: {
+      label: t("jobType.rescanLabel"),
+      description: t("jobType.rescanDesc"),
+      isThumbnailOnly: false,
+    },
     thumbnail_rebuild: {
       label: t("jobType.thumbnail_rebuildLabel"),
       description: t("jobType.thumbnail_rebuildDesc"),