feat(forum): plate follow cash coupons + free auto-order

- Meta _aksrt_follow_pay_coupon_s + plate admin toggle (money only)
- Plate_Follow_Pay soft/hard coupon apply, free-after-discount, burn via shared helper
- Cashier auto-submits free topic/plate orders; topic coupon label no longer "预留"

inc/classes/forum/class-aksrt-topic-meta-box.php

@@ -223,9 +223,9 @@ class AKSRT_Topic_Meta_Box {
         <p class="aksrt-topic-meta-pay-fields">
             <label>
                 <input type="checkbox" name="aksrt_topic_coupon_s" value="1" <?php checked($coupon_s); ?>>
-                <?php esc_html_e('允许优惠码（预留）', 'aksrt'); ?>
+                <?php esc_html_e('允许优惠码', 'aksrt'); ?>
             </label>
-            <span class="description" style="display:block;"><?php esc_html_e('标记本帖允许优惠码；收银台多网关接入前仅存 meta。', 'aksrt'); ?></span>
+            <span class="description" style="display:block;"><?php esc_html_e('开启后现金收银台可输入优惠码；积分解锁不吃券。', 'aksrt'); ?></span>
         </p>
         <?php
     }