fix(dto): fallback to legacy model in usage mapping

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

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

backend/internal/handler/dto/mappers.go

@@ -522,13 +522,17 @@ func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
 	// 普通用户 DTO：严禁包含管理员字段（例如 account_rate_multiplier、ip_address、account）。
 	requestType := l.EffectiveRequestType()
 	stream, openAIWSMode := service.ApplyLegacyRequestFields(requestType, l.Stream, l.OpenAIWSMode)
+	requestedModel := l.RequestedModel
+	if requestedModel == "" {
+		requestedModel = l.Model
+	}
 	return UsageLog{
 		ID:                    l.ID,
 		UserID:                l.UserID,
 		APIKeyID:              l.APIKeyID,
 		AccountID:             l.AccountID,
 		RequestID:             l.RequestID,
-		Model:                 l.RequestedModel,
+		Model:                 requestedModel,
 		ServiceTier:           l.ServiceTier,
 		ReasoningEffort:       l.ReasoningEffort,
 		InboundEndpoint:       l.InboundEndpoint,

backend/internal/handler/dto/mappers_usage_test.go

@@ -133,6 +133,21 @@ func TestUsageLogFromService_UsesRequestedModelAndKeepsUpstreamAdminOnly(t *test
 	require.Contains(t, string(adminJSON), `"upstream_model":"claude-sonnet-4-20250514"`)
 }
 
+func TestUsageLogFromService_FallsBackToLegacyModelWhenRequestedModelMissing(t *testing.T) {
+	t.Parallel()
+
+	log := &service.UsageLog{
+		RequestID: "req_legacy",
+		Model:     "claude-3",
+	}
+
+	userDTO := UsageLogFromService(log)
+	adminDTO := UsageLogFromServiceAdmin(log)
+
+	require.Equal(t, "claude-3", userDTO.Model)
+	require.Equal(t, "claude-3", adminDTO.Model)
+}
+
 func f64Ptr(value float64) *float64 {
 	return &value
 }