more subtle ways to test constraints

[mwotton]

currently, we recommend that there be a healthcheck endpoint that throws 500 if anything's broken. This is a barely-adequate stopgap for now: we probably want a way to pass in conditions that can access the full sequence of calls - something like certain routes always returning a 401 before authorisation occurs.

could look to https://hackage.haskell.org/package/servant-quickcheck-0.0.10.0 for inspiration here.