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.Api/Endpoints/YouTubeEndpoints.cs

@@ -40,7 +40,7 @@ public static class YouTubeEndpoints
         CancellationToken cancellationToken)
     {
         var result = await mediator.Send(
-            new GetYouTubeTrackStreamQuery(videoId, currentUser.RequiredId),
+            new ResolveYouTubeTrackStreamCommand(videoId, currentUser.RequiredId),
             cancellationToken);
 
         return result.ToHttpResult();