feat(api): 增强易码通数据导出逻辑以优化字段去重

在exports.go中新增逻辑，针对易码通数据处理时，保留order_voucher.grant_time字段并移除order_cash.receive_time字段，避免返回空值。此改动提升了字段去重的准确性，确保导出数据符合预期。

server/internal/api/exports.go

@@ -300,6 +300,22 @@ func (a *ExportsAPI) create(w http.ResponseWriter, r *http.Request) {
 			filtered = deduped
 		}
 	}
+	// 易码通立减金：保留 order_voucher.grant_time，移除红包领取时间列，避免“领取时间”为空
+	if ds == "ymt" && tv == 3 {
+		deduped := make([]string, 0, len(filtered))
+		removed := []string{}
+		for _, tf := range filtered {
+			if tf == "order_cash.receive_time" {
+				removed = append(removed, tf)
+				continue
+			}
+			deduped = append(deduped, tf)
+		}
+		if len(removed) > 0 {
+			logging.JSON("INFO", map[string]interface{}{"event": "fields_deduplicated_receive_time", "removed": removed, "reason": "立减金保留 order_voucher.grant_time"})
+		}
+		filtered = deduped
+	}
 	// 易码通客户名称字段去重：若同时选择 order.merchant_name 与 merchant.name，仅保留 merchant.name
 	if ds == "ymt" {
 		hasOrderMerchant := false