Implement directory privacy settings and add rate limiting

Adds configurable privacy options for member directories, allowing restrictions on visibility based on roles or login status. Introduces rate limiting for unauthenticated AJAX requests to prevent brute-force attacks or abuse.

includes/admin/class-secure.php

@@ -209,6 +209,13 @@ if ( ! class_exists( 'um\admin\Secure' ) ) {
 			$scanner_content .= '</span>';
 
 			$secure_fields = array(
+				array(
+					'id'             => 'ajax_nopriv_rate_limit',
+					'type'           => 'checkbox',
+					'label'          => __( 'Rate Limit', 'ultimate-member' ),
+					'checkbox_label' => __( 'Enable Rate Limiting', 'ultimate-member' ),
+					'description'    => __( 'This prevents brute-force enumeration attempts in guest AJAX requests.', 'ultimate-member' ),
+				),
 				array(
 					'id'               => 'banned_capabilities',
 					'type'             => 'multi_checkbox',