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)

DeepDrftData/Migrations/20260612000000_SoftDeleteOrphanedReleases.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DeepDrftData.Migrations
+{
+    /// <inheritdoc />
+    // Data-only migration: no schema change, snapshot unchanged.
+    public partial class SoftDeleteOrphanedReleases : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Backfill: soft-delete any live release whose tracks were all soft-deleted before the
+            // delete-cascade in UnifiedTrackService existed. These show as 0-track rows in the albums
+            // browser; this clears the pre-existing orphans the cascade now prevents going forward.
+            migrationBuilder.Sql(@"
+UPDATE release
+SET is_deleted = true,
+    updated_at = now()
+WHERE id IN (
+    SELECT r.id
+    FROM release r
+    WHERE r.is_deleted = false
+      AND NOT EXISTS (
+          SELECT 1
+          FROM track t
+          WHERE t.release_id = r.id
+            AND t.is_deleted = false
+      )
+);");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("-- no-op: orphaned release soft-deletes are not rolled back");
+        }
+    }
+}