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/shlink-install.sh

@@ -93,10 +93,7 @@ server {
     }
 }
 EOF
-  ln -sf /etc/nginx/sites-available/shlink-web-client /etc/nginx/sites-enabled/shlink-web-client
-  rm -f /etc/nginx/sites-enabled/default
-  systemctl enable -q nginx
-  $STD systemctl restart nginx
+  nginx_enable_site shlink-web-client
   msg_ok "Set up Web Client"
 fi