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/Mixes/GetMixById.cs

@@ -22,9 +22,10 @@ namespace Musify.Application.Mixes
             if (mix is null)
                 return Error.NotFound(description: $"Mix {request.MixId} was not found");
 
-            var items = mix.Items.OrderBy(item => item.Position).ToList();
+            var orderedItems = mix.Items.OrderBy(item => item.Position).ToList();
+            var items = await MixItemMapper.ToResponsesAsync(database, orderedItems, cancellationToken);
 
-            return MixApplicationResponse.FromEntity(mix, items.Count, items);
+            return new MixApplicationResponse(mix.Id, mix.Title, mix.Subtitle, items.Count, items);
         }
     }
 }