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.

service/tiered_settle.go

@@ -132,9 +132,19 @@ func PrepareTieredBillingForSelectedGroup(c *gin.Context, relayInfo *relaycommon
 			types.ErrOptionWithSkipRetry(),
 		)
 	}
-	if snap == nil || snap.GroupRatio == 0 {
+	if snap == nil {
 		return nil
 	}
+	if snap.GroupRatio == 0 {
+		// Paid-to-free keeps FreeModel as-is: FreeModel means "pre-consume was
+		// skipped", which is not true once a session exists, and settlement
+		// already yields 0 for a zero group ratio.
+		return nil
+	}
+
+	// The selected group is paid; clear a FreeModel flag frozen when the
+	// initial group was free so downstream state stays consistent.
+	relayInfo.PriceData.FreeModel = false
 
 	if relayInfo.Billing == nil {
 		return PreConsumeBilling(c, snap.EstimatedQuotaAfterGroup, relayInfo)