Routers and project structure

A single main.py is right for the first afternoon and wrong by the end of the week. Relay already has tickets, customers, a chat endpoint, and a health check, each with its own models and dependencies, and they do not belong in one file. FastAPI's unit of organisation is the APIRouter: a mini-application that holds a set of routes and is mounted into the real app with a prefix. The app file becomes a short assembly point, and each feature lives in its own module.

The second half of this lesson is the layout around the routers. A router should be thin: it parses the request, calls a service function, and shapes the response. The rules about tickets live in a service module that knows nothing about HTTP, which is what makes them testable and reusable from the agent later.