fix: sor_get_pay_feerate 加 ORDER BY + LIMIT 1 防非确定性取值

无 ORDER BY 时 MySQL 可能返回非确定的行顺序，
两次同参数调用取到不同费率。

unipay/payfee.py

@@ -59,6 +59,8 @@ where
 	and b.enabled_date <= ${biz_date}$
 	and b.expired_date >= ${biz_date}$
 	and a.id = ${channelid}$
+order by b.enabled_date desc
+limit 1
 """
 	ns = {
 		'channelid': channelid,