feat(pay): content cashier coupon and rebate discount chain

Wire zibll-style cash content discounts: VIP list price → buyer 推广折扣 → coupon, then commission on final pay. Reuse AKSRT_Coupon, add Invite::get_referrer, cashier UI, and paid-time coupon burn + rebate_log.

inc/classes/settings/class-aksrt-settings-home-cms.php

@@ -383,6 +383,10 @@ class AKSRT_Settings_Home_Cms {
         return AKSRT_Settings_Payment_Runtime::get_paid_content_settings();
     }
 
+    public static function get_content_coupon_settings(): array {
+        return AKSRT_Settings_Payment_Runtime::get_content_coupon_settings();
+    }
+
     public static function get_payment_recharge_settings(): array {
         return AKSRT_Settings_Payment_Runtime::get_payment_recharge_settings();
     }