feat: Newsletter system improvements and validation framework

- Fix: Marketing events now display in Staff notifications tab
- Reorganize: Move Coupons to Marketing/Coupons for better organization
- Add: Comprehensive email/phone validation with extensible filter hooks
  - Email validation with regex pattern (xxxx@xxxx.xx)
  - Phone validation with WhatsApp verification support
  - Filter hooks for external API integration (QuickEmailVerification, etc.)
- Fix: Newsletter template routes now use centralized notification email builder
- Add: Validation.php class for reusable validation logic
- Add: VALIDATION_HOOKS.md documentation with integration examples
- Add: NEWSLETTER_CAMPAIGN_PLAN.md architecture for future campaign system
- Fix: API delete method call in Newsletter.tsx (delete -> del)
- Remove: Duplicate EmailTemplates.tsx (using notification system instead)
- Update: Newsletter controller to use centralized Validation class

Breaking changes:
- Coupons routes moved from /routes/Coupons to /routes/Marketing/Coupons
- Legacy /coupons routes maintained for backward compatibility

includes/Api/NewsletterController.php

@@ -4,6 +4,7 @@ namespace WooNooW\API;
 use WP_REST_Request;
 use WP_REST_Response;
 use WP_Error;
+use WooNooW\Core\Validation;
 
 class NewsletterController {
     const API_NAMESPACE = 'woonoow/v1';
@@ -112,8 +113,11 @@ class NewsletterController {
     public static function subscribe(WP_REST_Request $request) {
         $email = sanitize_email($request->get_param('email'));
         
-        if (!is_email($email)) {
-            return new WP_Error('invalid_email', 'Invalid email address', ['status' => 400]);
+        // Use centralized validation with extensible filter hooks
+        $validation = Validation::validate_email($email, 'newsletter_subscribe');
+        
+        if (is_wp_error($validation)) {
+            return $validation;
         }
         
         // Get existing subscribers (now stored as objects with metadata)