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/feishin-install.sh

@@ -57,10 +57,7 @@ set +a
 envsubst </opt/feishin/settings.js.template >/etc/nginx/conf.d/settings.js
 envsubst '${PUBLIC_PATH}' </opt/feishin/ng.conf.template >/etc/nginx/sites-available/feishin
 
-ln -sf /etc/nginx/sites-available/feishin /etc/nginx/sites-enabled/feishin
-rm -f /etc/nginx/sites-enabled/default
-systemctl enable -q --now nginx
-systemctl reload nginx
+nginx_enable_site feishin
 msg_ok "Published Web Assets"
 
 motd_ssh