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/internal/modules/mohong/config.go

@@ -12,10 +12,8 @@ import (
 
 const (
 	configKeyDefaultQrcodeURL  = "mohong.default_qrcode_url"
-	configKeyGroupWelcomeText  = "mohong.group_welcome_text"
 	configKeyOrderCopyTemplate = "mohong.order_copy_template"
 
-	defaultGroupWelcomeText = "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。"
 	defaultOrderCopyTemplate = "【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}}）"
 )
 
@@ -23,7 +21,7 @@ func (r *Repository) GetConfig(ctx context.Context) (*ConfigDTO, error) {
 	if r.db == nil {
 		return nil, ErrDependencyUnavailable
 	}
-	keys := []string{configKeyDefaultQrcodeURL, configKeyGroupWelcomeText, configKeyOrderCopyTemplate}
+	keys := []string{configKeyDefaultQrcodeURL, configKeyOrderCopyTemplate}
 	var rows []model.SystemConfig
 	if err := r.db.WithContext(ctx).Where("`key` IN ?", keys).Find(&rows).Error; err != nil {
 		return nil, err
@@ -34,7 +32,6 @@ func (r *Repository) GetConfig(ctx context.Context) (*ConfigDTO, error) {
 	}
 	return &ConfigDTO{
 		DefaultQrcodeURL:  strings.TrimSpace(values[configKeyDefaultQrcodeURL]),
-		GroupWelcomeText:  firstNonEmpty(values[configKeyGroupWelcomeText], defaultGroupWelcomeText),
 		OrderCopyTemplate: firstNonEmpty(values[configKeyOrderCopyTemplate], defaultOrderCopyTemplate),
 	}, nil
 }
@@ -49,11 +46,6 @@ func (r *Repository) UpdateConfig(ctx context.Context, req UpdateConfigRequest)
 				return err
 			}
 		}
-		if req.GroupWelcomeText != nil {
-			if err := upsertConfig(tx, configKeyGroupWelcomeText, strings.TrimSpace(*req.GroupWelcomeText), "摸大红订单群欢迎语"); err != nil {
-				return err
-			}
-		}
 		if req.OrderCopyTemplate != nil {
 			if err := upsertConfig(tx, configKeyOrderCopyTemplate, strings.TrimSpace(*req.OrderCopyTemplate), "摸大红订单一键复制文案模板"); err != nil {
 				return err
@@ -100,14 +92,6 @@ func (r *Repository) resolveQrcodeURL(tx *gorm.DB, productQrcode string) string
 	return ""
 }
 
-func (r *Repository) loadWelcomeText(tx *gorm.DB) string {
-	var cfg model.SystemConfig
-	if err := tx.Where("`key` = ?", configKeyGroupWelcomeText).First(&cfg).Error; err == nil && strings.TrimSpace(cfg.Value) != "" {
-		return strings.TrimSpace(cfg.Value)
-	}
-	return defaultGroupWelcomeText
-}
-
 func (r *Repository) loadCopyTemplate(tx *gorm.DB) string {
 	var cfg model.SystemConfig
 	if err := tx.Where("`key` = ?", configKeyOrderCopyTemplate).First(&cfg).Error; err == nil && strings.TrimSpace(cfg.Value) != "" {