Clean up API

[DanielOaks]

We've been adding endpoints to do all kinds of things, and the structure of the API is a bit of a mess. Now would be a good time to clean up the API, probably while making up a nice simple OpenAPI file to document it.

[DanielOaks]

The OpenAPI file has a cleaned-up API structure that I'm fairly happy with. I think the issue currently is that we don't need to support all CRUD operations for images themselves, since the frontend focuses on category-image associations instead. The next step is probably performing this quick cleanup, and keeping the full restructure for when the admin interface exposes more capabilities.