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

customer-spa/src/types/product.ts

@@ -7,6 +7,7 @@ export interface ProductCategory {
   name: string;
   slug: string;
   count?: number;
+  term_id?: number;
 }
 
 export interface Product {
@@ -31,6 +32,11 @@ export interface Product {
   attributes?: any[];
   variations?: number[];
   permalink?: string;
+  related_ids?: number[];
+  virtual?: boolean;
+  downloadable?: boolean;
+  review_count?: number;
+  average_rating?: string;
 }
 
 export interface ProductsResponse {