feat: add import service and API layer (refs #26)

Implements Milestone 3: Backend import service and API with:

Service Layer (user-import.service.ts):
- generatePreview(): extract archive, validate, detect VIN conflicts
- executeMerge(): chunk-based import (100 records/batch), UPDATE existing by VIN, INSERT new via batchInsert
- executeReplace(): transactional DELETE all user data, batchInsert all records
- Conflict detection: VIN duplicates in vehicles
- Error handling: collect errors per record, continue, report in summary
- File handling: copy vehicle images and documents from archive to storage
- Cleanup: delete temp directory in finally block

API Layer:
- POST /api/user/import: multipart upload, mode selection (merge/replace)
- POST /api/user/import/preview: preview without executing import
- Authentication: fastify.authenticate preHandler
- Content-Type validation: application/gzip or application/x-gzip
- Magic byte validation: FileType.fromBuffer verifies tar.gz
- Request validation: Zod schema for mode selection
- Response: ImportResult with success, mode, summary, warnings

Files Created:
- backend/src/features/user-import/domain/user-import.service.ts
- backend/src/features/user-import/api/user-import.controller.ts
- backend/src/features/user-import/api/user-import.routes.ts
- backend/src/features/user-import/api/user-import.validation.ts

Files Updated:
- backend/src/app.ts: register userImportRoutes with /api prefix

Quality:
- Type-check: PASS (0 errors)
- Linting: PASS (0 errors, 470 warnings - all pre-existing)
- Repository pattern: snake_case→camelCase conversion
- User-scoped: all queries filter by user_id
- Transaction boundaries: Replace mode atomic, Merge mode per-batch

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

backend/src/app.ts

@@ -31,6 +31,7 @@ import { userProfileRoutes } from './features/user-profile';
 import { onboardingRoutes } from './features/onboarding';
 import { userPreferencesRoutes } from './features/user-preferences';
 import { userExportRoutes } from './features/user-export';
+import { userImportRoutes } from './features/user-import/api/user-import.routes';
 import { pool } from './core/config/database';
 import { configRoutes } from './core/config/config.routes';
 
@@ -92,7 +93,7 @@ async function buildApp(): Promise<FastifyInstance> {
       status: 'healthy',
       timestamp: new Date().toISOString(),
       environment: process.env['NODE_ENV'],
-      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
+      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export', 'user-import']
     });
   });
 
@@ -102,7 +103,7 @@ async function buildApp(): Promise<FastifyInstance> {
       status: 'healthy',
       scope: 'api',
       timestamp: new Date().toISOString(),
-      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export']
+      features: ['admin', 'auth', 'config', 'onboarding', 'vehicles', 'documents', 'fuel-logs', 'stations', 'maintenance', 'platform', 'notifications', 'user-profile', 'user-preferences', 'user-export', 'user-import']
     });
   });
 
@@ -143,6 +144,7 @@ async function buildApp(): Promise<FastifyInstance> {
   await app.register(userProfileRoutes, { prefix: '/api' });
   await app.register(userPreferencesRoutes, { prefix: '/api' });
   await app.register(userExportRoutes, { prefix: '/api' });
+  await app.register(userImportRoutes, { prefix: '/api' });
   await app.register(configRoutes, { prefix: '/api' });
 
   // 404 handler