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/lib/api.ts

@@ -433,7 +433,7 @@ export async function getThumbnailStats() {
 export type StatusMappingDto = {
   id: string;
   provider_status: string;
-  mapped_status: string;
+  mapped_status: string | null;
 };
 
 export async function fetchStatusMappings(): Promise<StatusMappingDto[]> {