fix: allow fastify to set content-type dynamically
We can set content-type: application/json for specific routes, but
doing so ends up with confusing, over-engineered code.
Instead we should take care when auditing the endpoints.
* test: allow mocking of env vars
Since utils/env is a module, we can mock it to control env vars in
tests. However, it's not compatible with building the server in
setupFilesAfterEnv, so, instead, we can use a utility function to keep
things DRY.
* fix: update type of fastifyTestInstance
* chore: add comment about sts preload
* chore: rename header plugin
* test: add get util + provide origin on request
* feat: add cors headers
* chore: add TODO
