Make backend the source of truth for API defaults

[pamelafox]

Currently, the frontend values control the default for parameters sent to the backend, which makes the defaults awkward to adjust, and makes it hard to send requests to the API in a way that mirrors the frontend.
It would be better if the backend had the defaults (like with Quart-Schema, with dataclasses or Pydantic models), and the frontend fetched those via the /config call and populated the Developer Settings form based on them.