debug: add comprehensive logging to vehicle import merge (refs #26)

Added detailed logging to diagnose import merge issues:

- Log vehicle count at merge start
- Log each vehicle being processed with VIN/make/model/year
- Log when existing vehicles are found (by VIN or license plate)
- Log successful vehicle creation with new vehicle ID
- Log errors with full context (userId, VIN, make, model, error message)
- Log merge completion with summary statistics

This will help diagnose why vehicles show as "successfully imported" but don't appear in the UI.

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

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

@@ -231,6 +231,8 @@ export class UserImportService {
   ): Promise<void> {
     const vehicles = await this.archiveService.readDataFile<any>(extractedPath, 'vehicles.json');
 
+    logger.info('Merge vehicles starting', { userId, vehicleCount: vehicles.length });
+
     if (vehicles.length === 0) {
       return;
     }
@@ -242,11 +244,22 @@ export class UserImportService {
 
       for (const vehicle of chunk) {
         try {
+          logger.debug('Processing vehicle', {
+            userId,
+            vin: vehicle.vin,
+            make: vehicle.make,
+            model: vehicle.model,
+            year: vehicle.year,
+          });
+
           let existing = null;
 
           // Try to find existing vehicle by VIN first
           if (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
@@ -255,10 +268,17 @@ export class UserImportService {
             existing = allUserVehicles.find(
               (v) => v.licensePlate && v.licensePlate.toLowerCase() === vehicle.licensePlate.toLowerCase()
             ) || null;
+            if (existing) {
+              logger.debug('Found existing vehicle by license plate', {
+                vehicleId: existing.id,
+                licensePlate: vehicle.licensePlate,
+              });
+            }
           }
 
           if (existing) {
             // Update existing vehicle
+            logger.debug('Updating existing vehicle', { vehicleId: existing.id });
             await this.vehiclesRepo.update(existing.id, {
               make: vehicle.make,
               model: vehicle.model,
@@ -278,7 +298,14 @@ export class UserImportService {
           }
 
           // Insert new vehicle using service (enforces tier limits)
-          await this.vehiclesService.createVehicle(
+          logger.debug('Creating new vehicle via service', {
+            userId,
+            vin: vehicle.vin || '',
+            make: vehicle.make,
+            model: vehicle.model,
+          });
+
+          const result = await this.vehiclesService.createVehicle(
             {
               vin: vehicle.vin || '',
               make: vehicle.make,
@@ -296,18 +323,34 @@ export class UserImportService {
             },
             userId
           );
+
+          logger.info('Vehicle created successfully', { vehicleId: result.id });
           summary.imported++;
         } catch (error) {
-          if (error instanceof VehicleLimitExceededError) {
+          const errorMsg =
-            summary.errors.push(
+            error instanceof VehicleLimitExceededError
-              `Vehicle limit exceeded: ${error.upgradePrompt} (current: ${error.currentCount}/${error.limit})`
+              ? `Vehicle limit exceeded: ${error.upgradePrompt} (current: ${error.currentCount}/${error.limit})`
-            );
+              : `Vehicle import failed: ${error instanceof Error ? error.message : String(error)}`;
-          } else {
+
-            summary.errors.push(`Vehicle import failed: ${error instanceof Error ? error.message : String(error)}`);
+          logger.error('Vehicle import error', {
-          }
+            userId,
+            vin: vehicle.vin,
+            make: vehicle.make,
+            model: vehicle.model,
+            error: errorMsg,
+          });
+
+          summary.errors.push(errorMsg);
         }
       }
     }
+
+    logger.info('Merge vehicles completed', {
+      userId,
+      imported: summary.imported,
+      updated: summary.updated,
+      errors: summary.errors.length,
+    });
   }
 
   /**