refactor(backend): unify YouTube track creation via a provisioner

Track get-or-create for YouTube videos was duplicated between the
old GetYouTubeTrackStream query handler (which also wrote to the
database) and AddYouTubeTrackToPlayList, with frontend-supplied
metadata (title/artist/duration/thumbnail) trusted at add-to-playlist
time instead of resolving it server-side.

Extract YouTubeTrackProvisioner as the single get-or-create path,
resolving structured artist refs (id + name) from YouTube Music so
multi-artist songs map to distinct Artist rows. Rename
GetYouTubeTrackStream to ResolveYouTubeTrackStream (it writes, so it
is a command, not a query) and delegate to the provisioner.
AddYouTubeTrackToPlayList slims down to (UserId, PlayListId, VideoId)
since all metadata now comes from the provisioner/YouTube API.

backend/Musify.Infrastructure/Services/YouTubeMusicService.cs

@@ -49,7 +49,12 @@ namespace Musify.Infrastructure.Services
                         .OrderByDescending(thumbnail => thumbnail.Width)
                         .Select(thumbnail => thumbnail.Url)
                         .FirstOrDefault() ?? string.Empty,
-                    song.IsExplicit))
+                    song.IsExplicit,
+                    song.Artists
+                        .Select(artist => new YouTubeArtistRef(
+                            string.IsNullOrEmpty(artist.Id) ? null : artist.Id,
+                            artist.Name))
+                        .ToList()))
                 .ToList();
 
             var nextToken = string.Empty;
@@ -124,7 +129,12 @@ namespace Musify.Infrastructure.Services
                         .OrderByDescending(thumbnail => thumbnail.Width)
                         .Select(thumbnail => thumbnail.Url)
                         .FirstOrDefault() ?? string.Empty,
-                    info.IsExplicit);
+                    info.IsExplicit,
+                    info.Artists
+                        .Select(artist => new YouTubeArtistRef(
+                            string.IsNullOrEmpty(artist.Id) ? null : artist.Id,
+                            artist.Name))
+                        .ToList());
             }
             catch (Exception exception)
             {