fix: replace-audio duration write now unconditional via SetDuration

UpdateDuration's null guard matched zero rows for tracks that already had a duration (all normally-uploaded tracks). Add SetDurationAsync/SetDuration/ITrackService.SetDuration with no null guard; fail on zero rows. ReplaceAudioAsync now calls SetDuration.

DeepDrftData/TrackManager.cs

@@ -276,6 +276,21 @@ public class TrackManager
         }
     }
 
+    public async Task<ResultContainer<int>> SetDuration(long id, double durationSeconds, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var affected = await Repository.SetDurationAsync(id, durationSeconds, cancellationToken);
+            if (affected == 0)
+                return ResultContainer<int>.CreateFailResult($"Duration write matched no rows for track {id}.");
+            return ResultContainer<int>.CreatePassResult(affected);
+        }
+        catch (Exception e)
+        {
+            return ResultContainer<int>.CreateFailResult(e.Message);
+        }
+    }
+
     public async Task<ResultContainer<TrackDto>> Create(TrackDto newTrack)
     {
         try