Add comprehensive database setup and user management system

- Implement PostgreSQL database schema with users and bookmarks tables
- Add database connection pooling with retry logic and error handling
- Create migration system with automatic schema initialization
- Add database CLI tools for management (init, status, validate, etc.)
- Include comprehensive error handling and diagnostics
- Add development seed data and testing utilities
- Implement health monitoring and connection pool statistics
- Create detailed documentation and troubleshooting guide

Database features:
- Users table with authentication fields and email verification
- Bookmarks table with user association and metadata
- Proper indexes for performance optimization
- Automatic timestamp triggers
- Transaction support with rollback handling
- Connection pooling (20 max connections, 30s idle timeout)
- Graceful shutdown handling

CLI commands available:
- npm run db:init - Initialize database
- npm run db:status - Check database status
- npm run db:validate - Validate schema
- npm run db:test - Run database tests
- npm run db:diagnostics - Full diagnostics

backend/.env.example

@@ -0,0 +1,33 @@
+# Server Configuration
+NODE_ENV=development
+PORT=3001
+
+# Database Configuration
+DB_HOST=localhost
+DB_PORT=5432
+DB_NAME=bookmark_manager
+DB_USER=your_db_user
+DB_PASSWORD=your_db_password
+DB_SSL=false
+
+# JWT Configuration
+JWT_SECRET=your_super_secret_jwt_key_here_make_it_long_and_random
+JWT_EXPIRES_IN=24h
+
+# Email Configuration
+EMAIL_HOST=smtp.gmail.com
+EMAIL_PORT=587
+EMAIL_SECURE=false
+EMAIL_USER=your_email@gmail.com
+EMAIL_PASSWORD=your_app_password
+EMAIL_FROM=your_email@gmail.com
+
+# Application Configuration
+ALLOWED_ORIGINS=http://localhost:3000,http://127.0.0.1:3000
+BASE_URL=http://localhost:3001
+
+# Security Configuration
+BCRYPT_SALT_ROUNDS=12
+RATE_LIMIT_WINDOW_MS=900000
+RATE_LIMIT_MAX_REQUESTS=100
+AUTH_RATE_LIMIT_MAX=5