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/DeleteAlbum.cs

@@ -16,7 +16,7 @@ namespace Musify.Application.Albums
     {
         public async ValueTask<ErrorOr<Success>> Handle(DeleteAlbumCommand request, CancellationToken cancellationToken)
         {
-            var album = await database.UserAlbums
+            var album = await database.Albums
                 .SingleOrDefaultAsync(a => a.Id == request.AlbumId, cancellationToken);
             if (album is null)
             {
@@ -30,7 +30,7 @@ namespace Musify.Application.Albums
                 return Error.Unauthorized();
             }
 
-            database.UserAlbums.Remove(album);
+            database.Albums.Remove(album);
 
             try
             {