fix: stopCards 停机时补充写入 stop_reason 和 stopped_at 字段

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/task/polling_handler.go

@@ -365,7 +365,7 @@ func (h *PollingHandler) stopCardByUsageExhausted(ctx context.Context, card *mod
 	updates := map[string]any{
 		"network_status": 0, // 停机
 		"stopped_at":     now,
-		"stop_reason":    "套餐流量耗尽自动停机",
+		"stop_reason":    constants.StopReasonTrafficExhausted,
 		"updated_at":     now,
 	}
 	if err := h.db.Model(&model.IotCard{}).
@@ -686,12 +686,13 @@ func (h *PollingHandler) stopCards(ctx context.Context, cards []*model.IotCard,
 			zap.String("iccid", card.ICCID),
 			zap.String("reason", "套餐超额"))
 
-		// 更新数据库：卡的网络状态
 		now := time.Now()
 		if err := h.db.Model(&model.IotCard{}).
 			Where("id = ?", card.ID).
 			Updates(map[string]any{
-				"network_status": 0, // 停机
+				"network_status": 0,
+				"stopped_at":     now,
+				"stop_reason":    constants.StopReasonTrafficExhausted,
 				"updated_at":     now,
 			}).Error; err != nil {
 			h.logger.Error("更新卡状态失败", zap.Uint("card_id", card.ID), zap.Error(err))
@@ -1018,7 +1019,7 @@ func (h *PollingHandler) HandleProtectConsistencyCheck(ctx context.Context, t *a
 		h.db.Model(&model.IotCard{}).Where("id = ?", card.ID).Updates(map[string]any{
 			"network_status": constants.NetworkStatusOffline,
 			"stopped_at":     time.Now(),
-			"stop_reason":    "保护期一致性检查自动停机",
+			"stop_reason":    constants.StopReasonProtectPeriod,
 		})
 		h.updateCardCache(ctx, card.ID, map[string]any{"network_status": constants.NetworkStatusOffline})
 		return nil