refactor(youtube): log service failures and decouple error messages

YouTubeMusicService swallowed the exception into an ErrorOr description
with no trace on the server side. Adds ILogger to the three operations.
The download activity no longer names yt-dlp in its messages, since the
downloader is an implementation detail behind IYouTubeDownloaderService.

backend/Musify.Infrastructure/MassTransit/Activities/Audio/DownloadYouTubeAudioActivity.cs

@@ -43,10 +43,10 @@ namespace Musify.Infrastructure.MassTransit.Activities.Audio
 
                 if (downloadResult.ExitCode != 0)
                     throw new InvalidOperationException(
-                        $"yt-dlp failed with exit code {downloadResult.ExitCode}: {downloadResult.StandardError}");
+                        $"YouTube audio download failed: {downloadResult.StandardError}");
 
                 if (!File.Exists(destinationPath))
-                    throw new InvalidOperationException($"yt-dlp did not produce the expected file {destinationPath}.");
+                    throw new InvalidOperationException($"YouTube audio download did not produce the expected file {destinationPath}.");
 
                 logger.LogInformation("Downloaded YouTube audio for video {VideoId} to {DestinationPath}",
                     executeContext.Arguments.VideoId, destinationPath);

backend/Musify.Infrastructure/Services/YouTubeMusicService.cs

@@ -1,5 +1,6 @@
 using ErrorOr;
 using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Logging;
 using Musify.Application.Contracts;
 using Musify.Infrastructure.Configuration;
 using YouTubeMusicAPI.Client;
@@ -9,7 +10,7 @@ using YouTubeMusicAPI.Pagination;
 
 namespace Musify.Infrastructure.Services
 {
-    public class YouTubeMusicService(YouTubeConfiguration configuration, IMemoryCache cache) : IYouTubeMusicService
+    public class YouTubeMusicService(YouTubeConfiguration configuration, IMemoryCache cache, ILogger<YouTubeMusicService> logger) : IYouTubeMusicService
     {
         private readonly YouTubeMusicClient client = new(geographicalLocation: configuration.GeographicalLocation);
 
@@ -34,6 +35,7 @@ namespace Musify.Infrastructure.Services
             }
             catch (Exception exception)
             {
+                logger.LogError(exception, "YouTube Music search failed for query '{Query}'", query);
                 return Error.Failure(description: $"YouTube Music search failed: {exception.Message}");
             }
 
@@ -87,6 +89,7 @@ namespace Musify.Infrastructure.Services
             }
             catch (Exception exception)
             {
+                logger.LogError(exception, "Failed to resolve YouTube stream for '{VideoId}'", videoId);
                 return Error.Failure(description: $"Failed to resolve YouTube stream for '{videoId}': {exception.Message}");
             }
 
@@ -138,6 +141,7 @@ namespace Musify.Infrastructure.Services
             }
             catch (Exception exception)
             {
+                logger.LogError(exception, "Failed to load YouTube song info for '{VideoId}'", videoId);
                 return Error.Failure(description: $"Failed to load YouTube song info for '{videoId}': {exception.Message}");
             }
         }