feat(platform): summarize mileage queries

vehicle-data-platform/apps/api/internal/platform/model.go

@@ -142,6 +142,14 @@ type DailyMileageRow struct {
 	AnomalySeverity string  `json:"anomalySeverity,omitempty"`
 }
 
+type MileageSummary struct {
+	VehicleCount         int     `json:"vehicleCount"`
+	RecordCount          int     `json:"recordCount"`
+	SourceCount          int     `json:"sourceCount"`
+	TotalMileageKm       float64 `json:"totalMileageKm"`
+	AverageMileagePerVIN float64 `json:"averageMileagePerVin"`
+}
+
 type QualityIssueRow struct {
 	VIN       string `json:"vin"`
 	Plate     string `json:"plate"`