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

DeepDrftModels/DTOs/TrackDto.cs

@@ -16,6 +16,7 @@ public class TrackDto : BaseModel
     public string TrackName { get; set; } = string.Empty;
     public string? OriginalFileName { get; set; }
     public int TrackNumber { get; set; } = 1;
+    public double? DurationSeconds { get; set; }
     public long? ReleaseId { get; set; }
     public ReleaseDto? Release { get; set; }
 }