feat: add Newsletter Campaigns backend infrastructure

- Add CampaignManager.php with CPT registration, CRUD, batch sending
- Add CampaignsController.php with 8 REST endpoints (list, create, get, update, delete, send, test, preview)
- Register newsletter_campaign event in EventRegistry for email template
- Initialize CampaignManager in Bootstrap.php
- Register routes in Routes.php

includes/Api/Routes.php

@@ -23,6 +23,7 @@ use WooNooW\Api\CustomersController;
 use WooNooW\Api\NewsletterController;
 use WooNooW\Api\ModulesController;
 use WooNooW\Api\ModuleSettingsController;
+use WooNooW\Api\CampaignsController;
 use WooNooW\Frontend\ShopController;
 use WooNooW\Frontend\CartController as FrontendCartController;
 use WooNooW\Frontend\AccountController;
@@ -125,6 +126,9 @@ class Routes {
             // Newsletter controller
             NewsletterController::register_routes();
             
+            // Campaigns controller
+            CampaignsController::register_routes();
+            
             // Modules controller
             $modules_controller = new ModulesController();
             $modules_controller->register_routes();