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.

assets/admin/src/schema/sections/payment/paid.js

@@ -23,6 +23,8 @@ export const paidSchema = {
     payment_download_post_log: 50,
     payment_download_remote_size: 2048,
     payment_download_remote_ext: 'zip,rar,7z',
+    coupon_post_s: false,
+    coupon_desc: '',
   },
   groups: [
     {
@@ -72,5 +74,14 @@ export const paidSchema = {
         { key: 'payment_download_remote_ext', label: '远程文件格式限制', type: 'input', default: 'zip,rar,7z', help: '逗号分隔扩展名', dependsOn: { key: 'payment_download_remote_s', value: true } },
       ],
     },
+    {
+      key: 'coupon',
+      title: '内容优惠码',
+      icon: 'fas fa-ticket-alt',
+      fields: [
+        { key: 'coupon_post_s', label: '全站内容允许优惠码', type: 'switch', default: false, help: '开启后所有付费内容收银台可用优惠码；关闭时仅单篇勾选「允许优惠码」的文章可用。' },
+        { key: 'coupon_desc', label: '优惠码说明', type: 'textarea', default: '', dependsOn: { key: 'coupon_post_s', value: true }, help: '展示在内容收银台优惠码输入框下方。' },
+      ],
+    },
   ],
 }