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.Application/Contracts/IYouTubeMusicService.cs

@@ -2,6 +2,8 @@ using ErrorOr;
 
 namespace Musify.Application.Contracts
 {
+    public record YouTubeArtistRef(string? Id, string Name);
+
     public record YouTubeSongResult(
         string VideoId,
         string Title,
@@ -9,7 +11,8 @@ namespace Musify.Application.Contracts
         string Album,
         int DurationSeconds,
         string ThumbnailUrl,
-        bool IsExplicit);
+        bool IsExplicit,
+        IReadOnlyList<YouTubeArtistRef> Artists);
 
     public record YouTubeSearchResult(
         IReadOnlyList<YouTubeSongResult> Items,