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-credits-payment-groups.php

@@ -136,6 +136,15 @@ trait AKSRT_Settings_Credits_Payment_Groups {
         return AKSRT_Settings_Payment_Runtime::get_paid_content_settings();
     }
 
+    /**
+     * Content coupon global switches.
+     *
+     * @return array{enabled:bool,desc:string}
+     */
+    public static function get_content_coupon_settings(): array {
+        return AKSRT_Settings_Payment_Runtime::get_content_coupon_settings();
+    }
+
     /**
      * @return array{enabled:bool,mode:string,packages_raw:string,package_count:int,help_text:string}
      */