Refactor to use FastAPI status constants

[danielballan]

Suggested by @DiamondJoseph (and I agree) we should use human-readable status code constants. https://fastapi.tiangolo.com/tutorial/response-status-code/#shortcut-to-remember-the-names

In fact these are defined in starlette so we can import them from there.

[padraic-shafer]

Amen. I use this on other projects, and appreciate the clarity in the moment.