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/GetTrackCoverQueryHandlerTests.cs

@@ -18,7 +18,7 @@ namespace Musify.Application.Tests.Tracks
         public async Task Handle_ProcessedTrack_ReturnsRequestedSize(string size, string expectedFileName)
         {
             var owner = TestEntities.User();
-            var track = TestEntities.LocalTrack(owner);
+            var track = TestEntities.Track(owner);
             await SeedAsync(owner, track);
 
             var result = await CreateHandler().Handle(new GetTrackCoverQuery(track.Id, size), TestContext.Current.CancellationToken);
@@ -41,7 +41,7 @@ namespace Musify.Application.Tests.Tracks
         public async Task Handle_PictureNotProcessedYet_ReturnsNotFound()
         {
             var owner = TestEntities.User();
-            var track = TestEntities.LocalTrack(owner, pictures: TestEntities.PendingPictures());
+            var track = TestEntities.Track(owner, pictures: TestEntities.PendingPictures());
             await SeedAsync(owner, track);
 
             var result = await CreateHandler().Handle(new GetTrackCoverQuery(track.Id, "small"), TestContext.Current.CancellationToken);