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/GetAlbumsByUserId.cs

@@ -23,7 +23,7 @@ namespace Musify.Application.Albums
             if (!userExists)
                 return Error.NotFound(description: $"User {request.UserId} not found");
 
-            var albumsQuery = database.UserAlbums
+            var albumsQuery = database.Albums
                 .AsNoTracking()
                 .Where(album => album.OwnerUserId == request.UserId);