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/Events/DownloadYouTubeTrackEvent.cs

@@ -1,14 +0,0 @@
-using Musify.Application.Shared;
-
-namespace Musify.Application.Events
-{
-    public record DownloadYouTubeTrackEvent(
-        Guid TrackId,
-        string VideoId,
-        string ThumbnailUrl,
-        string Bucket,
-        string AudioProcessedFolderKey,
-        ImageSize Small,
-        ImageSize Medium,
-        ImageSize Large);
-}