fix: ignore unknown provider statuses instead of storing them

normalize_series_status now returns None when no mapping exists,
so unknown provider statuses won't pollute series_metadata.status.
Users can see unmapped statuses in Settings and assign them before
they get stored.

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()) {
-        Some(crate::metadata::normalize_series_status(pool, raw).await)
+        crate::metadata::normalize_series_status(pool, raw).await
     } else {
         None
     };