securo: enable nginx site via nginx_enable_site helper

Replaces the manual symlink + default removal + restart with the core
helper, which also runs nginx -t before restarting and enables the unit.

install/securo-install.sh

@@ -156,9 +156,7 @@ server {
     }
 }
 EOF
-ln -sf /etc/nginx/sites-available/securo.conf /etc/nginx/sites-enabled/securo.conf
-rm -f /etc/nginx/sites-enabled/default
-systemctl restart nginx
+nginx_enable_site securo.conf
 msg_ok "Configured Nginx"
 
 motd_ssh