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/GetTrackById.cs

@@ -3,7 +3,6 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
 using Musify.Application.Tracks.Responses;
-using Musify.Domain.Entities;
 
 namespace Musify.Application.Tracks
 {
@@ -16,9 +15,7 @@ namespace Musify.Application.Tracks
         public async ValueTask<ErrorOr<TrackApplicationResponse>> Handle(GetTrackByIdQuery request, CancellationToken cancellationToken)
         {
             var entity = await database.Tracks.AsNoTracking()
-                .Include(t => ((ExternalTrack)t).TrackArtists)
-                    .ThenInclude(trackArtist => trackArtist.Artist)
-                .Include(t => ((LocalTrack)t).Owner)
+                .Include(t => t.Owner)
                 .Where(t => t.Id == request.TrackId)
                 .Select(t => new { Track = t, ListensCount = t.ListeningHistories.Count })
                 .SingleOrDefaultAsync(cancellationToken);