Implement database integration (PostgreSQL)

Add PostgreSQL support with connection pooling, migrations, and health checks

Acceptance Criteria

• Add PostgreSQL driver dependencies (tokio-postgres, sqlx, or diesel)
• Implement database connection configuration
• Set up connection pooling for optimal performance
• Create database migration system
• Implement basic CRUD operations examples
• Add database health check endpoint
• Handle database connection failures gracefully
• Add database-related environment variables
• Create Docker Compose setup with PostgreSQL service
• Document database setup and configuration
• Add database backup/restore documentation
• Implement proper error handling for database operations

Technical Notes

• Consider using sqlx for async database operations
• Implement connection pooling with configurable pool size
• Use database migrations for schema management
• Add proper logging for database operations
• Ensure database connections are properly closed
• Consider implementing database connection retry logic
• Add database performance monitoring