fix: refresh stale browse cache on track edits and allow deleting empty releases

- Add CmsTrackBrowserViewModel.Invalidate(); called from TrackEdit/BatchEdit on save or delete so album/genre cache is invalidated and re-fetches on next mode switch
- CmsAlbumBrowser now handles 0-track releases: confirm dialog + DeleteReleaseAsync instead of early return; partial-failure path also fires OnReleasesChanged to trigger cache invalidation
- TrackList.OnAlbumsChanged now calls VM.Invalidate() so genres stay fresh after any album delete
- UnifiedTrackService.DeleteAsync cascades release soft-delete when last live track is removed (non-fatal; logs on failure)
- New DELETE api/track/release/{id} endpoint (ApiKeyAuthorize) for direct release soft-delete
- EF migration SoftDeleteOrphanedReleases backfills existing orphaned release rows via raw SQL (data-only, no schema change)

DeepDrftManager/Services/CmsTrackBrowserViewModel.cs

@@ -70,4 +70,15 @@ public class CmsTrackBrowserViewModel
     {
         ExpandedGenre = ExpandedGenre == genre ? null : genre;
     }
+
+    /// <summary>
+    /// Drop the cached album and genre datasets so the next <see cref="SwitchModeAsync"/> into
+    /// either mode re-fetches from the API. Call after a track or release mutation (edit, delete)
+    /// since both datasets are derived from the catalogue and go stale on any such change.
+    /// </summary>
+    public void Invalidate()
+    {
+        Albums = Array.Empty<ReleaseDto>();
+        Genres = Array.Empty<GenreSummaryDto>();
+    }
 }