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/Core/Musify.Application/Tracks/GetTracksByUserId.cs

@@ -6,7 +6,6 @@ using Musify.Application.Tracks.Responses;
 using X.PagedList;
 using X.PagedList.EF;
 using Musify.Application.Contracts;
-using Musify.Domain.Entities;
 
 namespace Musify.Application.Tracks
 {
@@ -27,9 +26,7 @@ namespace Musify.Application.Tracks
 
             var tracksQuery = database.UserHasTracks
                 .AsNoTracking()
-                .Include(ut => ((ExternalTrack)ut.Track).TrackArtists)
-                    .ThenInclude(trackArtist => trackArtist.Artist)
-                .Include(ut => ((LocalTrack)ut.Track).Owner)
+                .Include(ut => ut.Track.Owner)
                 .Where(p => p.UserId == request.UserId);
 
             if (!string.IsNullOrEmpty(request.Name))