fix(backend): keep the saved cover art at full resolution

Search results now carry a small thumbnail, so the artwork downloaded when a
YouTube track is saved would have shrunk with them. The URL is upsized to the
configured artwork resolution right before the download event goes out.

backend/Musify.Infrastructure/Configuration/YouTubeConfiguration.cs

@@ -23,5 +23,8 @@ namespace Musify.Infrastructure.Configuration
 
         [Range(32, 720)]
         public int ThumbnailSize { get; set; } = 240;
+
+        [Range(120, 1080)]
+        public int ArtworkSize { get; set; } = 544;
     }
 }