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.Api/AppSettings.json

@@ -54,7 +54,8 @@
         "SearchContinuationCacheSeconds": 600,
         "SearchResultsCacheSeconds": 900,
         "SongInfoCacheSeconds": 86400,
-        "ThumbnailSize": 240
+        "ThumbnailSize": 240,
+        "ArtworkSize": 544
     },
     "PlayList": {
         "Routes": {

backend/Musify.Application/Contracts/IYouTubeMusicService.cs

@@ -27,5 +27,7 @@ namespace Musify.Application.Contracts
         Task<ErrorOr<YouTubeStreamInfo>> GetAudioStreamAsync(string videoId, CancellationToken cancellationToken);
 
         Task<ErrorOr<YouTubeSongResult>> GetSongAsync(string videoId, CancellationToken cancellationToken);
+
+        string ResolveArtworkUrl(string thumbnailUrl);
     }
 }

backend/Musify.Application/PlayLists/AddYouTubeTrackToPlayList.cs

@@ -111,13 +111,13 @@ namespace Musify.Application.PlayLists
         private async Task<ErrorOr<string>> ResolveThumbnailUrlAsync(string videoId, YouTubeSongResult? song, CancellationToken cancellationToken)
         {
             if (!string.IsNullOrEmpty(song?.ThumbnailUrl))
-                return song.ThumbnailUrl;
+                return youTubeMusicService.ResolveArtworkUrl(song.ThumbnailUrl);
 
             var songResult = await youTubeMusicService.GetSongAsync(videoId, cancellationToken);
             if (songResult.IsError)
                 return songResult.Errors;
 
-            return songResult.Value.ThumbnailUrl;
+            return youTubeMusicService.ResolveArtworkUrl(songResult.Value.ThumbnailUrl);
         }
 
         private async Task PublishDownloadEventAsync(Guid trackId, string videoId, string thumbnailUrl, CancellationToken cancellationToken)

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;
     }
 }

backend/Musify.Infrastructure/Services/YouTubeMusicService.cs

@@ -160,6 +160,9 @@ namespace Musify.Infrastructure.Services
             }
         }
 
+        public string ResolveArtworkUrl(string thumbnailUrl) =>
+            YouTubeThumbnail.WithSize(thumbnailUrl, configuration.ArtworkSize);
+
         private string PickThumbnail(IEnumerable<YouTubeMusicAPI.Models.Thumbnail> thumbnails)
         {
             var ordered = thumbnails.OrderBy(thumbnail => thumbnail.Width).ToList();