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/Tests/Musify.Application.Tests/Albums/GetAlbumCoverQueryHandlerTests.cs

@@ -17,7 +17,7 @@ namespace Musify.Application.Tests.Albums
         public async Task Handle_ExistingAlbum_ReturnsRequestedSize(string size, string expectedFileName)
         {
             var owner = TestEntities.User();
-            var album = TestEntities.UserAlbum(owner.Id);
+            var album = TestEntities.Album(owner.Id);
             await SeedAsync(owner, album);
 
             var result = await CreateHandler().Handle(new GetAlbumCoverQuery(album.Id, size), TestContext.Current.CancellationToken);