移除摸大红支付后建群逻辑，仅保留扫码与复制

支付成功只固化客服二维码与订单文案；同步清理迁移、配置与前后端相关字段。

backend/migrations/000028_mohong.sql

@@ -21,7 +21,7 @@ CREATE TABLE IF NOT EXISTS mohong_products (
   KEY idx_mohong_products_status_sort (status, sort_order, id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='摸大红商品';
 
--- 摸大红订单表
+-- 摸大红订单表（支付后展示固定二维码 + 可复制文案，不建群）
 CREATE TABLE IF NOT EXISTS mohong_orders (
   id BIGINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
   order_no VARCHAR(64) NOT NULL COMMENT '订单号',
@@ -34,7 +34,6 @@ CREATE TABLE IF NOT EXISTS mohong_orders (
   product_snapshot JSON NULL COMMENT '商品快照',
   qrcode_url_snapshot VARCHAR(512) NOT NULL DEFAULT '' COMMENT '支付时解析的二维码快照',
   copy_text TEXT NOT NULL COMMENT '可复制订单文案',
-  conversation_id BIGINT UNSIGNED NULL COMMENT '订单群会话ID',
   paid_at DATETIME NULL COMMENT '支付时间',
   completed_at DATETIME NULL COMMENT '完成时间',
   cancelled_at DATETIME NULL COMMENT '取消时间',
@@ -48,15 +47,9 @@ CREATE TABLE IF NOT EXISTS mohong_orders (
   KEY idx_mohong_orders_status_created (status, created_at)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='摸大红订单';
 
--- 会话表增加摸大红订单关联
-ALTER TABLE chat_conversations
-  ADD COLUMN mohong_order_id BIGINT UNSIGNED NULL COMMENT '摸大红订单ID' AFTER listing_id,
-  ADD UNIQUE KEY uk_chat_conversations_mohong_order_id (mohong_order_id);
-
 -- 业务配置
 INSERT INTO system_configs (`key`, `value`, description) VALUES
 ('mohong.default_qrcode_url', '', '摸大红全局默认固定二维码图片URL'),
-('mohong.group_welcome_text', '订单已支付，请扫码联系客服，并将下方订单信息复制发送给客服。', '摸大红订单群欢迎语'),
 ('mohong.order_copy_template', '【摸大红订单】\n订单号：{{order_no}}\n下单时间：{{created_at}}\n商品：{{product_title}}\n数量：{{quantity}}\n金额：{{amount}}\n下单人：{{buyer_name}}（{{buyer_phone}}）', '摸大红订单一键复制文案模板')
 ON DUPLICATE KEY UPDATE description = VALUES(description);
 
@@ -111,14 +104,9 @@ DELETE FROM permissions WHERE code IN (
 
 DELETE FROM system_configs WHERE `key` IN (
   'mohong.default_qrcode_url',
-  'mohong.group_welcome_text',
   'mohong.order_copy_template'
 );
 
-ALTER TABLE chat_conversations
-  DROP INDEX uk_chat_conversations_mohong_order_id,
-  DROP COLUMN mohong_order_id;
-
 DROP TABLE IF EXISTS mohong_orders;
 DROP TABLE IF EXISTS mohong_products;