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/PlayLists/GetPlayListTracksQueryHandlerTests.cs

@@ -15,8 +15,8 @@ namespace Musify.Application.Tests.PlayLists
         {
             var owner = TestEntities.User();
             var playList = TestEntities.PlayList(owner.Id);
-            var second = TestEntities.LocalTrack(owner, "Second");
-            var first = TestEntities.LocalTrack(owner, "First");
+            var second = TestEntities.Track(owner, "Second");
+            var first = TestEntities.Track(owner, "First");
             await SeedAsync(
                 owner, playList, second, first,
                 new PlayListHasTrack { PlayListId = playList.Id, TrackId = second.Id, Position = 1 },