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/Responses/TrackSearchItemResponse.cs

@@ -1,22 +1,7 @@
-using Musify.Application.Contracts;
-using Musify.Domain.ValueObjects;
-
 namespace Musify.Application.Tracks.Responses
 {
-    /// <summary>One item of a combined local/YouTube track listing. Exactly one of
-    /// <see cref="Track"/> or <see cref="YouTubeSong"/> is populated: <see cref="Track"/> for
-    /// anything already in the catalog (local uploads and YouTube tracks provisioned earlier),
-    /// <see cref="YouTubeSong"/> for a live YouTube Music search hit that has not been
-    /// provisioned yet, so it has no track id.</summary>
-    public record TrackSearchItemResponse(
-        TrackSource Source,
-        TrackApplicationResponse? Track,
-        YouTubeSongResult? YouTubeSong)
+    public record TrackSearchItemResponse(TrackApplicationResponse Track)
     {
-        public static TrackSearchItemResponse FromTrack(TrackApplicationResponse track) =>
-            new(track is ExternalTrackApplicationResponse ? TrackSource.YouTube : TrackSource.Local, track, null);
-
-        public static TrackSearchItemResponse FromYouTubeSong(YouTubeSongResult song) =>
-            new(TrackSource.YouTube, null, song);
+        public static TrackSearchItemResponse FromTrack(TrackApplicationResponse track) => new(track);
     }
 }