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/Validators/PlayLists/AddYouTubeTrackRequestValidator.cs

@@ -10,20 +10,5 @@ public sealed class AddYouTubeTrackRequestValidator : AbstractValidator<AddYouTu
         RuleFor(x => x.VideoId)
             .NotEmpty()
             .Matches("^[A-Za-z0-9_-]{11}$");
-
-        RuleFor(x => x.Title)
-            .NotEmpty()
-            .MaximumLength(200);
-
-        RuleFor(x => x.Artist)
-            .MaximumLength(400);
-
-        RuleFor(x => x.DurationSeconds)
-            .GreaterThan(0);
-
-        RuleFor(x => x.ThumbnailUrl)
-            .NotEmpty()
-            .Must(url => Uri.TryCreate(url, UriKind.Absolute, out var uri) && uri.Scheme == Uri.UriSchemeHttps)
-            .WithMessage("ThumbnailUrl must be an absolute https URL.");
     }
 }