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/Hosts/Musify.Api/DataTransferObjects/PlayLists/AddPlayListTrackRequest.cs

@@ -1,8 +1,3 @@
-using Musify.Domain.ValueObjects;
-
 namespace Musify.Api.DataTransferObjects.PlayLists;
 
-/// <summary>Add a track already in the catalog by <see cref="TrackId"/>, or one from an external
-/// source by <see cref="Source"/> + <see cref="ExternalId"/> (provisioning it in the background if
-/// needed) — exactly one of the two must be given.</summary>
-public record AddPlayListTrackRequest(Guid? TrackId, TrackSource? Source, string? ExternalId);
+public record AddPlayListTrackRequest(Guid TrackId);