fix: preserve vehicle identity by checking ID first in merge mode (refs #26)

Critical fix for merge mode vehicle matching logic.

Problem:
- Vehicles with same license plate but no VIN were matched to the same existing vehicle
- Example: 2 vehicles with license plate "TEST-123" both updated the same vehicle
- Result: "Updated: 2" but only 1 vehicle in database, second vehicle overwrites first

Root Cause:
- Matching order was: VIN → license plate
- Both vehicles had no VIN and same license plate
- Both matched the same existing vehicle by license plate

Solution:
- New matching order: ID → VIN → license plate
- Preserves vehicle identity across export/import cycles
- Vehicles exported with IDs will update the same vehicle on re-import
- New vehicles (no matching ID) will be created as new records
- Security check: Verify ID belongs to same user before matching

Benefits:
- Export-modify-import workflow now works correctly
- Vehicles maintain identity across imports
- Users can safely import data with duplicate license plates
- Prevents unintended overwrites

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

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

@@ -246,23 +246,41 @@ export class UserImportService {
         try {
           logger.debug('Processing vehicle', {
             userId,
+            id: vehicle.id,
             vin: vehicle.vin,
             make: vehicle.make,
             model: vehicle.model,
             year: vehicle.year,
+            licensePlate: vehicle.licensePlate,
           });
 
           let existing = null;
 
-          // Try to find existing vehicle by VIN first
+          // Try to find existing vehicle by ID first (preserves identity across exports)
-          if (vehicle.vin && vehicle.vin.trim().length > 0) {
+          if (vehicle.id) {
+            existing = await this.vehiclesRepo.findById(vehicle.id);
+            // Verify it belongs to the same user
+            if (existing && existing.userId !== userId) {
+              logger.warn('Vehicle ID belongs to different user, ignoring', {
+                vehicleId: vehicle.id,
+                expectedUserId: userId,
+                actualUserId: existing.userId,
+              });
+              existing = null;
+            } else if (existing) {
+              logger.debug('Found existing vehicle by ID', { vehicleId: existing.id });
+            }
+          }
+
+          // Try to find existing vehicle by VIN if not found by ID
+          if (!existing && vehicle.vin && vehicle.vin.trim().length > 0) {
             existing = await this.vehiclesRepo.findByUserAndVIN(userId, vehicle.vin.trim());
             if (existing) {
               logger.debug('Found existing vehicle by VIN', { vehicleId: existing.id, vin: vehicle.vin });
             }
           }
 
-          // If not found by VIN and license plate exists, try license plate
+          // If not found by ID or 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(