新增摸大红业务并修复后台链接按钮白字

支持分类筛选、搜索、下单支付建群与固定二维码；合并迁移种子数据；后台表格编辑/详情链接恢复主题色可见。

backend/internal/modules/mohong/errors.go

@@ -0,0 +1,16 @@
+package mohong
+
+import "errors"
+
+var (
+	ErrDependencyUnavailable = errors.New("dependency unavailable")
+	ErrProductNotFound       = errors.New("product not found")
+	ErrProductUnavailable    = errors.New("product unavailable")
+	ErrStockInsufficient     = errors.New("stock insufficient")
+	ErrInvalidRequest        = errors.New("invalid request")
+	ErrOrderNotFound         = errors.New("order not found")
+	ErrOrderCannotPay        = errors.New("order cannot pay")
+	ErrOrderCannotCancel     = errors.New("order cannot cancel")
+	ErrOrderCannotComplete   = errors.New("order cannot complete")
+	ErrUnauthorized          = errors.New("unauthorized")
+)