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/linkding-install.sh

@@ -115,10 +115,8 @@ server {
     }
 }
 EOF
-$STD rm -f /etc/nginx/sites-enabled/default
-$STD ln -sf /etc/nginx/sites-available/linkding /etc/nginx/sites-enabled/linkding
-systemctl enable -q --now nginx linkding linkding-tasks
-systemctl restart nginx
+systemctl enable -q --now linkding linkding-tasks
+nginx_enable_site linkding
 msg_ok "Created Services"
 
 motd_ssh