fix(usage): preserve requested model in gateway billing paths

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

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

backend/internal/service/openai_gateway_service.go

@@ -4110,9 +4110,9 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		multiplier = resolver.Resolve(ctx, user.ID, *apiKey.GroupID, apiKey.Group.RateMultiplier)
 	}
 
-	billingModel := result.Model
+	billingModel := forwardResultBillingModel(result.Model, result.UpstreamModel)
 	if result.BillingModel != "" {
-		billingModel = result.BillingModel
+		billingModel = strings.TrimSpace(result.BillingModel)
 	}
 	serviceTier := ""
 	if result.ServiceTier != nil {
@@ -4140,6 +4140,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		AccountID:             account.ID,
 		RequestID:             requestID,
 		Model:                 result.Model,
+		RequestedModel:        result.Model,
 		UpstreamModel:         optionalNonEqualStringPtr(result.UpstreamModel, result.Model),
 		ServiceTier:           result.ServiceTier,
 		ReasoningEffort:       result.ReasoningEffort,