feat: add maintenance cost aggregation for TCO (refs #15)

- Add MaintenanceCostStats interface
- Add getVehicleMaintenanceCosts() method to maintenance service
- Validates numeric cost values and throws on invalid data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/maintenance/domain/maintenance.service.ts

@@ -9,7 +9,8 @@ import type {
   MaintenanceRecordResponse,
   MaintenanceScheduleResponse,
   MaintenanceCategory,
-  ScheduleType
+  ScheduleType,
+  MaintenanceCostStats
 } from './maintenance.types';
 import { validateSubtypes } from './maintenance.types';
 import { MaintenanceRepository } from '../data/maintenance.repository';
@@ -63,6 +64,19 @@ export class MaintenanceService {
     return records.map(r => this.toRecordResponse(r));
   }
 
+  async getVehicleMaintenanceCosts(vehicleId: string, userId: string): Promise<MaintenanceCostStats> {
+    const records = await this.repo.findRecordsByVehicleId(vehicleId, userId);
+    const totalCost = records.reduce((sum, r) => {
+      if (r.cost === null || r.cost === undefined) return sum;
+      const cost = Number(r.cost);
+      if (isNaN(cost)) {
+        throw new Error(`Invalid cost value for maintenance record ${r.id}`);
+      }
+      return sum + cost;
+    }, 0);
+    return { totalCost, recordCount: records.length };
+  }
+
   async updateRecord(userId: string, id: string, patch: UpdateMaintenanceRecordRequest): Promise<MaintenanceRecordResponse | null> {
     const existing = await this.repo.findRecordById(id, userId);
     if (!existing) return null;

backend/src/features/maintenance/domain/maintenance.types.ts

@@ -162,6 +162,12 @@ export interface MaintenanceRecordResponse extends MaintenanceRecord {
   subtypeCount: number;
 }
 
+// TCO aggregation stats
+export interface MaintenanceCostStats {
+  totalCost: number;
+  recordCount: number;
+}
+
 export interface MaintenanceScheduleResponse extends MaintenanceSchedule {
   subtypeCount: number;
   isDueSoon?: boolean;