fix: prevent vehicle duplication and enforce tier limits in merge mode (refs #26)

Critical bug fixes for import merge mode:

1. Vehicle duplication bug (RULE 0 - CRITICAL):
   - Previous: Vehicles without VINs always inserted as new, creating duplicates
   - Fixed: Check by VIN first, then fallback to license plate matching
   - Impact: Prevents duplicate vehicles on repeated imports

2. Vehicle limit bypass (RULE 0 - CRITICAL):
   - Previous: Direct repo.create() bypassed tier-based vehicle limits
   - Fixed: Use VehiclesService.createVehicle() which enforces FOR UPDATE locking and tier checks
   - Impact: Free users properly limited to 1 vehicle, prevents limit violations

Changes:
- Added VehiclesService to import service constructor
- Updated mergeVehicles() to check VIN then license plate for matches
- Replace repo.create() with service.createVehicle() for limit enforcement
- Added VehicleLimitExceededError handling with clear error messages

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

backend/src/features/user-import/domain/user-import.service.ts

@@ -9,6 +9,7 @@ import { Pool, PoolClient } from 'pg';
 import { logger } from '../../../core/logging/logger';
 import { getStorageService } from '../../../core/storage/storage.service';
 import { VehiclesRepository } from '../../vehicles/data/vehicles.repository';
+import { VehiclesService, VehicleLimitExceededError } from '../../vehicles/domain/vehicles.service';
 import { FuelLogsRepository } from '../../fuel-logs/data/fuel-logs.repository';
 import { DocumentsRepository } from '../../documents/data/documents.repository';
 import { MaintenanceRepository } from '../../maintenance/data/maintenance.repository';
@@ -18,6 +19,7 @@ import { ImportPreview, ImportResult, USER_IMPORT_CONFIG } from './user-import.t
 export class UserImportService {
   private readonly archiveService: UserImportArchiveService;
   private readonly vehiclesRepo: VehiclesRepository;
+  private readonly vehiclesService: VehiclesService;
   private readonly fuelLogsRepo: FuelLogsRepository;
   private readonly maintenanceRepo: MaintenanceRepository;
   private readonly documentsRepo: DocumentsRepository;
@@ -26,6 +28,7 @@ export class UserImportService {
   constructor(private pool: Pool) {
     this.archiveService = new UserImportArchiveService();
     this.vehiclesRepo = new VehiclesRepository(pool);
+    this.vehiclesService = new VehiclesService(this.vehiclesRepo, pool);
     this.fuelLogsRepo = new FuelLogsRepository(pool);
     this.maintenanceRepo = new MaintenanceRepository(pool);
     this.documentsRepo = new DocumentsRepository(pool);
@@ -219,7 +222,7 @@ export class UserImportService {
   }
 
   /**
-   * Merge vehicles: UPDATE existing by VIN, INSERT new
+   * Merge vehicles: UPDATE existing by VIN or license plate, INSERT new with limit enforcement
    */
   private async mergeVehicles(
     userId: string,
@@ -239,9 +242,20 @@ export class UserImportService {
 
       for (const vehicle of chunk) {
         try {
-          // Check if vehicle exists by VIN
+          let existing = null;
+
+          // Try to find existing vehicle by VIN first
           if (vehicle.vin && vehicle.vin.trim().length > 0) {
-            const existing = await this.vehiclesRepo.findByUserAndVIN(userId, vehicle.vin.trim());
+            existing = await this.vehiclesRepo.findByUserAndVIN(userId, vehicle.vin.trim());
+          }
+
+          // If not found by VIN and license plate exists, try license plate
+          if (!existing && vehicle.licensePlate && vehicle.licensePlate.trim().length > 0) {
+            const allUserVehicles = await this.vehiclesRepo.findByUserId(userId);
+            existing = allUserVehicles.find(
+              (v) => v.licensePlate && v.licensePlate.toLowerCase() === vehicle.licensePlate.toLowerCase()
+            ) || null;
+          }
 
           if (existing) {
             // Update existing vehicle
@@ -262,20 +276,39 @@ export class UserImportService {
             summary.updated++;
             continue;
           }
-          }
 
-          // Insert new vehicle
-          await this.vehiclesRepo.create({
-            ...vehicle,
-            userId,
-          });
+          // Insert new vehicle using service (enforces tier limits)
+          await this.vehiclesService.createVehicle(
+            {
+              vin: vehicle.vin || '',
+              make: vehicle.make,
+              model: vehicle.model,
+              year: vehicle.year,
+              engine: vehicle.engine,
+              transmission: vehicle.transmission,
+              trimLevel: vehicle.trimLevel,
+              driveType: vehicle.driveType,
+              fuelType: vehicle.fuelType,
+              nickname: vehicle.nickname,
+              color: vehicle.color,
+              licensePlate: vehicle.licensePlate,
+              odometerReading: vehicle.odometerReading,
+            },
+            userId
+          );
           summary.imported++;
         } catch (error) {
+          if (error instanceof VehicleLimitExceededError) {
+            summary.errors.push(
+              `Vehicle limit exceeded: ${error.upgradePrompt} (current: ${error.currentCount}/${error.limit})`
+            );
+          } else {
             summary.errors.push(`Vehicle import failed: ${error instanceof Error ? error.message : String(error)}`);
           }
         }
       }
     }
+  }
 
   /**
    * Merge fuel logs: batch insert new records