Add user database service and unit tests

- Implement user database service with functions for user management (create, find, update, delete).
- Add comprehensive unit tests for user database service using Vitest.
- Mock database interactions to ensure isolated testing.
- Create setup files for unit tests to handle database connections and global mocks.
- Introduce error handling for unique constraints and foreign key violations.
- Enhance logging for better traceability during database operations.

src/services/backgroundJobService.test.ts

@@ -1,12 +1,12 @@
 // src/services/backgroundJobService.test.ts
 import { describe, it, expect, vi, beforeEach, type Mocked } from 'vitest';
 import cron from 'node-cron';
-import * as db from './db';
+import * as db from './db/index.db';
 import { logger } from './logger.server';
 import { sendDealNotificationEmail } from './emailService.server';
 import { runDailyDealCheck, startBackgroundJobs } from './backgroundJobService';
 import { WatchedItemDeal } from '../types';
-import { AdminUserView } from './db/admin';
+import { AdminUserView } from './db/admin.db';
 
 // Mock dependencies
 vi.mock('node-cron');