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-resource-meta-box.php

@@ -307,6 +307,26 @@ class AKSRT_Resource_Meta_Box {
                     </div>
                 </div>
 
+                <div class="aksrt-pmb__row">
+                    <label><?php esc_html_e('优惠码', 'aksrt'); ?></label>
+                    <div>
+                        <div style="display:flex; align-items:center; gap:6px; margin-bottom:8px;">
+                            <input type="checkbox" id="aksrt_pay_coupon_s" name="aksrt_pay_coupon_s" value="1" <?php checked(!empty($meta['coupon_s'])); ?>>
+                            <label for="aksrt_pay_coupon_s" style="font-weight:400;padding:0;"><?php esc_html_e('允许使用优惠码（全站开关关闭时仍可对本篇启用）', 'aksrt'); ?></label>
+                        </div>
+                        <textarea id="aksrt_pay_coupon_desc" name="aksrt_pay_coupon_desc" style="min-height:60px;" placeholder="<?php esc_attr_e('本篇优惠码说明，可留空', 'aksrt'); ?>"><?php echo esc_textarea((string) ($meta['coupon_desc'] ?? '')); ?></textarea>
+                        <p class="aksrt-pmb__hint"><?php esc_html_e('对齐 zibll coupon_s / coupon_desc。积分支付不使用优惠码。', 'aksrt'); ?></p>
+                    </div>
+                </div>
+
+                <div class="aksrt-pmb__row">
+                    <label for="aksrt_pay_rebate_discount"><?php esc_html_e('推广优惠金额', 'aksrt'); ?></label>
+                    <div>
+                        <input type="number" id="aksrt_pay_rebate_discount" name="aksrt_pay_rebate_discount" value="<?php echo esc_attr((string) ($meta['pay_rebate_discount'] ?? 0)); ?>" min="0" step="0.01">
+                        <p class="aksrt-pmb__hint"><?php esc_html_e('有有效推荐人且开启推广返佣时，现金购买可先减此金额（买家侧推广折扣，对齐 zibll pay_rebate_discount）。0 = 不启用。', 'aksrt'); ?></p>
+                    </div>
+                </div>
+
                 <div class="aksrt-pmb__row">
                     <label for="aksrt_pay_stock"><?php esc_html_e('库存', 'aksrt'); ?></label>
                     <div>
@@ -506,6 +526,9 @@ class AKSRT_Resource_Meta_Box {
             'free_preview'    => $_POST['aksrt_pay_free_preview'] ?? '',
             'show_sales'      => !empty($_POST['aksrt_pay_show_sales']),
             'download_limit_over_price' => max(0, (float) ($_POST['aksrt_pay_limit_over_price'] ?? 0)),
+            'coupon_s'            => !empty($_POST['aksrt_pay_coupon_s']),
+            'coupon_desc'         => (string) ($_POST['aksrt_pay_coupon_desc'] ?? ''),
+            'pay_rebate_discount' => max(0, (float) ($_POST['aksrt_pay_rebate_discount'] ?? 0)),
         ]);
 
         update_post_meta($post_id, AKSRT_Domain_Model::PAY_META_KEY, $pay_meta);