> does it then allow you to write custom code to add business logic

Yes! Like gavinray said, you can create HTTP services in any language of your choice. Hasura can then be configured to call HTTP endpoints when something changes in the DB. And your services can call Hasura anytime of course.

Remember that Hasura handles all the authorization for you, so there's no need to "proxy" requests made to it.

You end up writing very little compared to traditional frameworks.