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/ValueObjects/MixItemSource.cs

@@ -1,8 +0,0 @@
-namespace Musify.Domain.ValueObjects
-{
-    public enum MixItemSource
-    {
-        Musify = 0,
-        YouTube = 1
-    }
-}

backend/Core/Musify.Domain/ValueObjects/TrackSource.cs

@@ -1,8 +0,0 @@
-namespace Musify.Domain.ValueObjects
-{
-    public enum TrackSource
-    {
-        Local = 0,
-        YouTube = 1
-    }
-}