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/class-aksrt-invite.php

@@ -293,6 +293,57 @@ class AKSRT_Invite {
      * 公开 API：根据邀请码反查邀请人 user_id（不存在返回 0）。
      * 给注册流程在 wp_create_user 之前做"邀请码是否合法"的预校验用。
      */
+    /**
+     * Resolve affiliate / invite referrer for a buyer (zibll-style).
+     *
+     * Priority:
+     * 1. Logged-in: inviter_id from aksrt_invite table for this invitee
+     * 2. Cookie invite code → resolve_code
+     *
+     * Respects pay_rebate_self_s when available (via affiliate settings).
+     */
+    public function get_referrer(int $user_id = 0): int {
+        $uid = $user_id > 0 ? $user_id : get_current_user_id();
+        $referrer_id = 0;
+
+        if ($uid > 0) {
+            global $wpdb;
+            $table = AKSRT_DB_Migrator::table('invite');
+            if ($table) {
+                $referrer_id = (int) $wpdb->get_var($wpdb->prepare(
+                    "SELECT inviter_id FROM {$table} WHERE invitee_id = %d AND inviter_id > 0 ORDER BY id DESC LIMIT 1",
+                    $uid
+                ));
+            }
+        }
+
+        if ($referrer_id <= 0) {
+            $code = '';
+            if (!empty($_COOKIE[self::COOKIE_NAME])) {
+                $code = sanitize_key((string) $_COOKIE[self::COOKIE_NAME]);
+            }
+            if ($code !== '') {
+                $referrer_id = $this->resolve_code($code);
+            }
+        }
+
+        if ($referrer_id <= 0) {
+            return 0;
+        }
+
+        // Self-purchase rebate gate (default allow, matching pay_rebate_self_s default true).
+        $allow_self = true;
+        if (class_exists('AKSRT_Settings') && method_exists('AKSRT_Settings', 'get_payment_affiliate_settings')) {
+            $aff = AKSRT_Settings::get_payment_affiliate_settings();
+            $allow_self = !isset($aff['self_s']) || !empty($aff['self_s']);
+        }
+        if ($uid > 0 && $referrer_id === $uid && !$allow_self) {
+            return 0;
+        }
+
+        return $referrer_id;
+    }
+
     public function resolve_code(string $code): int {
         $code = sanitize_key($code);
         if ($code === '') return 0;