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).

deploy/compose.yml

@@ -447,7 +447,6 @@ services:
       DOTNET_ENVIRONMENT: ${DOTNET_ENVIRONMENT}
       AudioTranscoder__Routes__WorkingDirectory: /tmp/musify
       Workers__Routes__TemporaryFilesDirectory: /tmp/musify
-      YtDlp__AdditionalArguments: ${YTDLP_ADDITIONAL_ARGUMENTS:-}
     restart: unless-stopped
 
   # YARP in front of the SeaweedFS filer: validates the RS256 stream ticket and