fix(billing): harden tiered retry group-switch billing (#6570)

Follow-up to #6518 (issue #6480) addressing three review findings:

- Document and lock in arrears semantics for the wallet Reserve top-up:
  when an auto-group retry lands on a more expensive group, the full
  reservation delta is deducted unconditionally (balance may go
  negative), mirroring settlement, so the logged pre-consumed quota
  always reconciles with the actual balance movement. Genuine DB
  errors still fail the attempt with update_data_error. Subscription
  funding keeps its insufficient-quota behavior: subscriptions enforce
  a hard used<=total cap and do not support arrears.
- PriceData.FreeModel is cleared when a retry switches from a free
  group to a paid one, keeping it consistent with the billing session
  created at that point.
- getChannel refreshes GroupRatioInfo only after channel selection
  succeeds, and the retry loop records the channel in use_channel
  before PrepareTieredBillingForSelectedGroup can fail.

controller/relay.go

@@ -199,12 +199,12 @@ func Relay(c *gin.Context, relayFormat types.RelayFormat) {
 			newAPIError = channelErr
 			break
 		}
+		addUsedChannel(c, channel.Id)
 		if billingErr := service.PrepareTieredBillingForSelectedGroup(c, relayInfo); billingErr != nil {
 			newAPIError = billingErr
 			break
 		}
 
-		addUsedChannel(c, channel.Id)
 		bodyStorage, bodyErr := common.GetBodyStorage(c)
 		if bodyErr != nil {
 			// Ensure consistent 413 for oversized bodies even when error occurs later (e.g., retry path)
@@ -312,9 +312,6 @@ func getChannel(c *gin.Context, info *relaycommon.RelayInfo, retryParam *service
 		}, nil
 	}
 	channel, selectGroup, err := service.CacheGetRandomSatisfiedChannel(retryParam)
-
-	info.PriceData.GroupRatioInfo = helper.HandleGroupRatio(c, info)
-
 	if err != nil {
 		return nil, types.NewError(fmt.Errorf("获取分组 %s 下模型 %s 的可用渠道失败（retry）: %s", selectGroup, info.OriginModelName, err.Error()), types.ErrorCodeGetChannelFailed, types.ErrOptionWithSkipRetry())
 	}
@@ -322,6 +319,8 @@ func getChannel(c *gin.Context, info *relaycommon.RelayInfo, retryParam *service
 		return nil, types.NewError(fmt.Errorf("分组 %s 下模型 %s 的可用渠道不存在（retry）", selectGroup, info.OriginModelName), types.ErrorCodeGetChannelFailed, types.ErrOptionWithSkipRetry())
 	}
 
+	info.PriceData.GroupRatioInfo = helper.HandleGroupRatio(c, info)
+
 	newAPIError := middleware.SetupContextForSelectedChannel(c, channel, info.OriginModelName)
 	if newAPIError != nil {
 		return nil, newAPIError