Build a FastAPI summarization service

Implement a minimal but production-ready FastAPI service for text summarization. Requirements: - Expose an endpoint `POST /summarize`. - The request body is JSON: `{ "text": string }`. - The response body is JSON: `{ "summary": string }`. - The summarization logic can be a placeholder function; you do not need to call a real model. - Store each request and response pair in memory. Engineering requirements: - Use a clean structure with separate router, service, and model layers. - Use type hints throughout. - Use Pydantic models for request and response validation. - Handle invalid input and unexpected errors properly. - Keep the code modular, readable, and simple. - Do not use a database; in-memory storage is sufficient. What to provide: - Show the proposed file structure. - Then provide the code for each file.