chore: add SvelteKit nginx config, clean up Flask upstream, wire AE_APP_REPLICAS

- Add conf/nginx/site-enabled_aether_app_svelte_node.conf (SvelteKit node upstream)
- Clean up site-enabled_aether_flask_gunicorn.conf: remove dead comments, update
  upstream to app-node:3000
- docker-compose.yml: ae_app scale now uses ${AE_APP_REPLICAS:-1} instead of hardcoded 1

conf/nginx/site-enabled_aether_flask_gunicorn.conf

@@ -162,22 +162,6 @@ server {
 
 
 upstream flask_backend {
-    # sticky sessions
     ip_hash;
-
-    # enable least connections balancing method
-    # least_conn;
-
-    # zone backend 64k;  # Use NGINX Plus' shared memory
-
-    # server webserver1 weight=1;
-    # server webserver2 weight=4;
-
-    # larger number will recieve more requests
-    # Example of 20 vs 10: 20 will recieve twice as many requests as 10
-    server aether_app_gunicorn:5005 weight=20 max_fails=3 fail_timeout=30s;
-    # server aether_app_gunicorn_bak:5005 weight=10 max_fails=1 fail_timeout=30s;
-
-    # maintain up to 20 idle connections to the group of upstream servers
-    # keepalive 20;
+    server app-node:3000 weight=20 max_fails=3 fail_timeout=30s;
 }