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_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
 }