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/Tracks/CreateTrackCommandHandlerTests.cs

@@ -48,11 +48,11 @@ namespace Musify.Application.Tests.Tracks
 
             Assert.False(result.IsError);
             Assert.Equal("My Song", result.Value.Title);
-            var localResponse = Assert.IsType<LocalTrackApplicationResponse>(result.Value);
+            var localResponse = Assert.IsType<TrackApplicationResponse>(result.Value);
             Assert.Equal(user.Id, localResponse.OwnerUserId);
             await eventBus.Received(1).PublishAsync(Arg.Any<Musify.Application.Events.CreateTrackResourcesEvent>(), Arg.Any<CancellationToken>());
 
-            var stored = await Database.LocalTracks.FindAsync([result.Value.Id], TestContext.Current.CancellationToken);
+            var stored = await Database.Tracks.FindAsync([result.Value.Id], TestContext.Current.CancellationToken);
             Assert.NotNull(stored);
             Assert.Equal(ProcessingStatus.Pending, stored.Audio.TranscodeStatus);
         }