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>

infra/migrations/0042_nullable_mapped_status.sql

@@ -0,0 +1,3 @@
+-- Allow mapped_status to be NULL to represent "known but unmapped" provider statuses.
+-- Clicking X in the UI will set mapped_status to NULL instead of deleting the row.
+ALTER TABLE status_mappings ALTER COLUMN mapped_status DROP NOT NULL;