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.Infrastructure/MassTransit/Consumers/DownloadYouTubeTrackConsumer.cs

@@ -1,26 +0,0 @@
-using MassTransit;
-using Musify.Application.Events;
-using Musify.Infrastructure.MassTransit.RoutingSlip.Builders;
-
-namespace Musify.Infrastructure.MassTransit.Consumers
-{
-    public class DownloadYouTubeTrackConsumer(
-        IBus bus,
-        YouTubeTrackRoutingSlipBuilder routingSlipBuilder)
-        : IConsumer<DownloadYouTubeTrackEvent>
-    {
-        public async Task Consume(ConsumeContext<DownloadYouTubeTrackEvent> consumeContext)
-        {
-            var audioRoutingSlip = routingSlipBuilder
-                .BuildAudioWorkflow(consumeContext.Message, consumeContext.CorrelationId)
-                .Build();
-
-            var pictureRoutingSlip = routingSlipBuilder
-                .BuildPictureWorkflow(consumeContext.Message, consumeContext.CorrelationId)
-                .Build();
-
-            await bus.Execute(audioRoutingSlip);
-            await bus.Execute(pictureRoutingSlip);
-        }
-    }
-}