fix: unmap status mappings instead of deleting, store unmapped provider statuses

- Make mapped_status nullable so unmapping (X button) sets NULL instead of
  deleting the row — provider statuses never disappear from the UI
- normalize_series_status now returns the raw provider status (lowercased)
  when no mapping exists, so all statuses are stored in series_metadata
- Fix series_statuses query crash caused by NULL mapped_status values
- Fix metadata batch/refresh server actions crashing page on 400 errors
- StatusMappingDto.mapped_status is now string | null in the backoffice

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

apps/backoffice/app/libraries/page.tsx

@@ -80,7 +80,12 @@ export default async function LibrariesPage() {
   async function batchMetadataAction(formData: FormData) {
     "use server";
     const id = formData.get("id") as string;
-    await startMetadataBatch(id);
+    try {
+      await startMetadataBatch(id);
+    } catch {
+      // Library may have metadata disabled — ignore silently
+      return;
+    }
     revalidatePath("/libraries");
     revalidatePath("/jobs");
   }