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/ITrackService.cs

@@ -47,6 +47,14 @@ public interface ITrackService
     /// </summary>
     Task<ResultContainer<int>> UpdateDuration(long id, double durationSeconds, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Unconditionally overwrite the SQL duration for one track. Unlike <see cref="UpdateDuration"/>,
+    /// this carries no null guard — it is for the replace-audio path where the track already has a
+    /// non-null duration that must be overwritten with the new audio's value. Returns a fail Result
+    /// when zero rows are affected (track removed between lookup and write).
+    /// </summary>
+    Task<ResultContainer<int>> SetDuration(long id, double durationSeconds, CancellationToken cancellationToken = default);
+
     /// <summary>
     /// Resolve the release matching <paramref name="title"/> + <paramref name="artist"/>, creating
     /// one from <paramref name="releaseData"/> when none exists. Backs the upload flow's FK