Standardize nginx setup in install scripts

Refactors many install scripts to use the shared `nginx_enable_site` helper instead of manual symlink/default-site/reload sequences, making webserver setup more consistent and less error-prone. It also replaces hardcoded PHP-FPM socket paths with `get_php_fpm_socket` (including templated substitutions where needed) across nginx and caddy configs to improve compatibility with varying PHP versions and socket locations.

install/discourse-install.sh

@@ -207,10 +207,7 @@ server {
   }
 }
 EOF
-ln -sf /etc/nginx/sites-available/discourse /etc/nginx/sites-enabled/discourse
-rm -f /etc/nginx/sites-enabled/default
-$STD systemctl enable --now nginx
-$STD systemctl reload nginx
+nginx_enable_site discourse
 msg_ok "Configured Nginx"
 
 motd_ssh