Design a stock order manager

Design an order management service for stock buy and sell requests. The system does not match trades itself. Instead, it integrates with a third-party brokerage API: - `POST /orders` -> submits an order and returns a unique external order ID - `GET /orders/{id}` -> returns the latest order details and status - `DELETE /orders/{id}` -> requests cancellation of an existing order Your design should support: - Placing buy and sell orders - Tracking order status over time - Canceling orders - Handling orders that remain in a `pending` state for a long time - Handling orders that expire before being filled - Identifying likely system bottlenecks and explaining how to mitigate them Focus especially on order lifecycle management, correctness under retries or partial failures, and how the system stays consistent with the third-party brokerage.