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/Repositories/TrackRepository.cs

@@ -210,6 +210,18 @@ public class TrackRepository : Repository<DeepDrftContext, TrackEntity>
                 .SetProperty(t => t.DurationSeconds, durationSeconds)
                 .SetProperty(t => t.UpdatedAt, DateTime.UtcNow), ct);
 
+    // Unconditional duration overwrite for one track (no load round-trip), used by the replace-audio
+    // path. Unlike UpdateDurationAsync, there is no null guard — replace always overwrites the
+    // existing value because a normally-uploaded track already has a non-null DurationSeconds and the
+    // null-guarded backfill query would match zero rows and silently leave it stale. Returns the count
+    // of rows affected; zero means the track was removed between the GetById lookup and this write.
+    public async Task<int> SetDurationAsync(long id, double durationSeconds, CancellationToken ct = default)
+        => await Query
+            .Where(t => t.Id == id)
+            .ExecuteUpdateAsync(s => s
+                .SetProperty(t => t.DurationSeconds, durationSeconds)
+                .SetProperty(t => t.UpdatedAt, DateTime.UtcNow), ct);
+
     // Resolve an existing release by its natural key (title + artist). Returns null when no match,
     // signalling the manager to create one. Soft-deleted releases never match.
     public async Task<ReleaseEntity?> GetReleaseByTitleAndArtistAsync(