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/api/src/metadata_refresh.rs

@@ -575,7 +575,7 @@ async fn sync_series_with_diff(
     let new_start_year = candidate.start_year;
     let new_total_volumes = candidate.total_volumes;
     let new_status = if let Some(raw) = candidate.metadata_json.get("status").and_then(|s| s.as_str()) {
-        crate::metadata::normalize_series_status(pool, raw).await
+        Some(crate::metadata::normalize_series_status(pool, raw).await)
     } else {
         None
     };