Remove YouTube integration from backend and deployment config

Drops the YouTube-backed second catalog source entirely: the
Local/External split on Track/Album collapses into single concrete
entities, Artist/TrackArtist/ExternalTrack/ExternalAlbum and the
YouTube service/download pipeline (yt-dlp, MassTransit consumer,
routing slip) are deleted, and mixes now source discovery only from
local listening history. Includes the RemoveYouTubeIntegration EF
Core migration and matching Worker/deploy/compose cleanup (no more
yt-dlp/python/deno in the Worker image, no thumbnail-cache/YtDlp
config sections).

backend/Core/Musify.Application/Albums/CreateAlbum.cs

@@ -50,7 +50,7 @@ namespace Musify.Application.Albums
             var intent = pictureIntent.Value;
             var title = request.Title.Trim();
 
-            var album = new UserAlbum
+            var album = new Album
             {
                 OwnerUserId = request.UserId,
                 Title = title,
@@ -66,7 +66,7 @@ namespace Musify.Application.Albums
                 }
             };
 
-            await database.UserAlbums.AddAsync(album, cancellationToken);
+            await database.Albums.AddAsync(album, cancellationToken);
 
             var finalPictureKey = albumConfiguration.Routes.BuildOriginalPicturePath(request.UserId, intent.ObjectName);
             var publishResult = await PublishCreateAlbumEventAsync(album.Id, intent, finalPictureKey, cancellationToken);