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).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Tests/Musify.Application.Tests/Albums/CreateAlbumCommandHandlerTests.cs

@@ -44,7 +44,7 @@ namespace Musify.Application.Tests.Albums
             Assert.Equal(owner.Id, result.Value.OwnerUserId);
             Assert.Equal(0, result.Value.TrackCount);
 
-            var stored = await Database.UserAlbums.FindAsync([result.Value.Id], TestContext.Current.CancellationToken);
+            var stored = await Database.Albums.FindAsync([result.Value.Id], TestContext.Current.CancellationToken);
             Assert.NotNull(stored);
             Assert.Equal("MY ALBUM", stored.NormalizedTitle);
             Assert.Equal("cover.webp", stored.Pictures.OriginalName);