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/features/user-import/api/user-import.validation.ts

@@ -0,0 +1,12 @@
+/**
+ * @ai-summary Validation schemas for user import API
+ * @ai-context Zod schemas for import request validation
+ */
+
+import { z } from 'zod';
+
+export const importRequestSchema = z.object({
+  mode: z.enum(['merge', 'replace']).optional(),
+});
+
+export type ImportRequest = z.infer<typeof importRequestSchema>;