Add types to untyped code

This code has no types: <paste>

Add them incrementally:
- Start with the boundaries — function inputs and outputs, and the
  shape of anything coming from an API or a database.
- Types that describe reality, including nullable fields and the
  shape of an error response. Don't pretend everything is always
  present.
- Where the real type is genuinely dynamic, say so explicitly rather
  than using a permissive escape hatch quietly.

List every place you had to widen a type or use an escape hatch, and
why. Those are the places my code is doing something I should look at.