feat(exporter): 添加短信渠道字段的SQL构建逻辑

在sqlbuilder.go中新增短信渠道字段的SQL构建逻辑，支持将短信渠道值映射为对应的中文描述，提升导出数据的可读性和准确性。

server/internal/exporter/sqlbuilder.go

@@ -144,6 +144,11 @@ func BuildSQL(req BuildRequest, whitelist map[string]bool) (string, []interface{
 				cols = append(cols, "CASE `"+mt+"`.order_type WHEN 1 THEN '直充' WHEN 2 THEN '卡密' ELSE '' END AS `order_digit.order_type`")
 				continue
 			}
+			if f == "sms_channel" {
+				// 短信渠道枚举：1=官方，2=专票
+				cols = append(cols, "CASE `"+mt+"`.sms_channel WHEN 1 THEN '官方' WHEN 2 THEN '专票' ELSE '' END AS `order_digit.sms_channel`")
+				continue
+			}
 		}
 		if t == "order_cash" && f == "receive_status" {
 			cols = append(cols, "CASE `order_cash`.receive_status WHEN 0 THEN '待领取' WHEN 1 THEN '领取中' WHEN 2 THEN '领取成功' WHEN 3 THEN '领取失败' ELSE '' END AS `order_cash.receive_status`")