refactor(backend): return AudioTranscodeResult with status code and duration

backend/Musify.Application/Contracts/AudioTranscodeResult.cs

@@ -0,0 +1,4 @@
+namespace Musify.Application.Contracts
+{
+    public record AudioTranscodeResult(int StatusCode, TimeSpan Duration);
+}

backend/Musify.Application/Contracts/IAudioTranscoderService.cs

@@ -2,7 +2,7 @@ namespace Musify.Application.Contracts
 {
     public interface IAudioTranscoderService
     {
-        Task<TimeSpan> TranscodeToAudioFileAsync(Stream audioStream, string destinationPath, CancellationToken cancellationToken);
+        Task<AudioTranscodeResult> TranscodeToAudioFileAsync(Stream audioStream, string destinationPath, CancellationToken cancellationToken);
 
         Task<bool> IsValidAudioFileAsync(string filePath, CancellationToken cancellationToken);
     }