E-commerce Platform Backend

Design a backend-like model with User (Customer, Admin), a Product hierarchy, Order, and PaymentGateway.

Apply composition for cart logic. Use a Factory to create orders, and ensure extensibility for new product categories or payment methods.

Handle edge cases like refunds and inventory.

OOP Focus: Full system design with OOP principles.

Note: This is statement-only in the current Practice version; stdout should be empty.