[spectral](https://github.com/stoplightio/spectral) lints
openapi/swagger files for mistakes of which it has identified a few and
which I've fixed.
I had to put it into `lint-frontend` because it depends on node_modules
so can not run on Drone during the backend target. I plan to refactor
these targets later to `lint-js` and `lint-go` so that they are
categorized based on the tool dependencies.
