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/TrackApplicationResponse.cs

@@ -1,24 +1,11 @@
-using System.Text.Json.Serialization;
+using Musify.Application.Serialization;
 using Musify.Domain.Entities;
 using Musify.Domain.ValueObjects;
+using System.Text.Json.Serialization;
 
 namespace Musify.Application.Tracks.Responses
 {
-    // See AlbumApplicationResponse for why [JsonPolymorphic]/[JsonDerivedType] are required
-    // (not optional) whenever a base type carries subtype-only data across the wire. Track is a
-    // cleaner case than Album: both OwnerUserId (local-only) and ExternalId (external-only) are
-    // genuinely exclusive to one side, so the base has no "meaningless sentinel" field at all —
-    // every real value is one of the two derived types, hence base is abstract.
-    //
-    // No explicit Source property here (unlike the domain entities): STJ's polymorphism support
-    // already injects a "source" discriminator property with the same "Local"/"YouTube" values a
-    // hand-written TrackSource field would have, and a real property with that exact name would
-    // collide with it ("contains property 'source' that conflicts with an existing metadata
-    // property name"). Use the discriminator (or a type check) instead of a Source field.
-    [JsonPolymorphic(TypeDiscriminatorPropertyName = "source")]
-    [JsonDerivedType(typeof(LocalTrackApplicationResponse), typeDiscriminator: "Local")]
-    [JsonDerivedType(typeof(ExternalTrackApplicationResponse), typeDiscriminator: "YouTube")]
-    public abstract record TrackApplicationResponse(
+    public record TrackApplicationResponse(
         Guid Id,
         string Title,
         string? Artist,
@@ -26,24 +13,19 @@ namespace Musify.Application.Tracks.Responses
         double Duration,
         int ListensCount,
         DateTime CreatedAt,
-        DateTime UpdatedAt)
+        DateTime UpdatedAt,
+        [property: JsonConverter(typeof(LongAsStringConverter))] long OwnerUserId)
     {
         public static TrackApplicationResponse FromEntity(Track track, int listensCount) =>
-            track switch
-            {
-                LocalTrack local => LocalTrackApplicationResponse.FromEntity(local, listensCount),
-                ExternalTrack external => ExternalTrackApplicationResponse.FromEntity(external, listensCount),
-                _ => throw new NotSupportedException($"Unsupported track entity type '{track.GetType().Name}'.")
-            };
-
-        protected static string? FormatArtist(Track track) => track switch
-        {
-            LocalTrack local => local.Owner.Name,
-            ExternalTrack { TrackArtists.Count: > 0 } external =>
-                string.Join(", ", external.TrackArtists
-                    .OrderBy(trackArtist => trackArtist.Position)
-                    .Select(trackArtist => trackArtist.Artist.Name)),
-            _ => null
-        };
+            new(
+                track.Id,
+                track.Title,
+                track.Owner.Name,
+                track.Audio.TranscodeStatus,
+                track.DurationSeconds,
+                listensCount,
+                track.CreatedAt,
+                track.UpdatedAt,
+                track.OwnerUserId);
     }
 }