Build a food delivery system

• Restaurants have menus of items with prices and availability; customers browse and add items to a cart restricted to a single restaurant, with quantity updates and a total including taxes and a delivery fee
• placeOrder(customerId) converts the cart to an order with a snapshot of item names and prices, so later menu changes do not alter it
• The order follows a state machine: placed, accepted (or rejected) by the restaurant, preparing, ready, picked up, delivered, with cancellation allowed only in early states; invalid transitions are rejected
• A delivery agent is assigned when the order becomes ready (or accepted) using a pluggable strategy (nearest available agent, or least recently assigned); an agent holds at most one active order, and is freed on delivery
• If no agent is available, the order waits in a queue and is assigned when one frees up
• Observers (customer, restaurant, agent) receive status change events, and order history per customer is available

Only after every requirement works.

• Estimated delivery time from distance and preparation time
• Ratings for restaurant and agent
• Surge delivery fee based on demand