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

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