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/Core/Notifications/EventRegistry.php

@@ -63,6 +63,21 @@ class EventRegistry {
             'wc_email' => '',
             'enabled' => true,
         ],
+        'newsletter_campaign' => [
+            'id' => 'newsletter_campaign',
+            'label' => __('Newsletter Campaign', 'woonoow'),
+            'description' => __('Master email design template for newsletter campaigns', 'woonoow'),
+            'category' => 'marketing',
+            'recipient_type' => 'customer',
+            'wc_email' => '',
+            'enabled' => true,
+            'variables' => [
+                '{content}' => __('Campaign content', 'woonoow'),
+                '{campaign_title}' => __('Campaign title', 'woonoow'),
+                '{subscriber_email}' => __('Subscriber email', 'woonoow'),
+                '{unsubscribe_url}' => __('Unsubscribe link', 'woonoow'),
+            ],
+        ],
 
         // ===== ORDER INITIATION =====
         'order_placed' => [