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.Domain/Entities/Track.cs

@@ -1,11 +1,12 @@
 using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
 using Musify.Domain.Abstractions;
 using Musify.Domain.ValueObjects;
 
 namespace Musify.Domain.Entities
 {
 #pragma warning disable CS8618
-    public abstract class Track : IAuditable
+    public class Track : IAuditable
     {
         [Key]
         public Guid Id { get; set; } = Guid.NewGuid();
@@ -21,6 +22,12 @@ namespace Musify.Domain.Entities
         [Required]
         public double DurationSeconds { get; set; }
 
+        [Required]
+        public required long OwnerUserId { get; set; }
+
+        [ForeignKey(nameof(OwnerUserId))]
+        public required User Owner { get; set; }
+
         [Required]
         public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;