add openapi spec to document api

[edaniszewski]

instead of hand-crafting the docs for the api, we should be using openapi. theres a bunch of support for generating it, so it could be defined in-code.

tbd what the scale of this issue is. it could mean migrating to fastapi which has that well-supported and built-in natively, or it could mean finding/writing some tool/integration with sanic to allow us to do the same in its current state. second option is less work, but requires some research.