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/nginx/templates/default.conf.template

@@ -9,12 +9,6 @@
 
 resolver 127.0.0.11 valid=10s;
 
-# YouTube artwork the web player proxies from Google. Caching it on disk keeps
-# repeated searches off both the node process and Google, which answers 429
-# once a client pulls a whole grid of covers at a time.
-proxy_cache_path /var/cache/nginx/thumbnails levels=1:2 keys_zone=thumbnails:10m
-                 max_size=${MUSIFY_THUMBNAIL_CACHE_SIZE} inactive=30d use_temp_path=off;
-
 map $http_upgrade $connection_upgrade {
     default upgrade;
     ''      close;
@@ -40,21 +34,6 @@ server {
     ssl_certificate     /etc/nginx/certs/origin.pem;
     ssl_certificate_key /etc/nginx/certs/origin.key;
 
-    location = /api/youtube/thumbnail {
-        set $upstream http://web-player:3000;
-        proxy_pass $upstream;
-        include /etc/nginx/snippets/proxy.conf;
-
-        proxy_cache thumbnails;
-        proxy_cache_key $uri$is_args$args;
-        proxy_cache_valid 200 30d;
-        proxy_cache_valid 400 404 5m;
-        proxy_cache_lock on;
-        proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
-        proxy_cache_background_update on;
-        add_header X-Cache-Status $upstream_cache_status;
-    }
-
     location / {
         set $upstream http://web-player:3000;
         proxy_pass $upstream;