Wire NowPlayingStats to live aggregates: add SQL track duration column, stats endpoint, and duration backfill

DeepDrftModels/Entities/TrackEntity.cs

@@ -15,6 +15,10 @@ public class TrackEntity : BaseEntity, IEntity
     public required string TrackName { get; set; }
     public string? OriginalFileName { get; set; }
     public int TrackNumber { get; set; } = 1;
+    // Audio runtime in seconds, extracted by the processor at upload (AudioBinary.Duration) and
+    // persisted here so aggregate queries (e.g. total mix runtime) read it from SQL rather than the
+    // vault. Nullable: rows that predate this column are valid until the one-time backfill populates them.
+    public double? DurationSeconds { get; set; }
     public long? ReleaseId { get; set; }
     public ReleaseEntity? Release { get; set; }
 }