Rigorous API testing

[rien]

The integration tests are very slow because after each test the database is reset. Unless Rocket/Diesel has support transactional testing (every test is performed within a transaction which can be rollback'ed), we should consider using an alternative testing strategy.

I think we need full system tests here: setting up a production like environment with real data and launching actual HTTP API requests (without clearing the database each time). This does not need to be programmed within Rust, but can be a separate tool or script which is ergonomic to use.

As we want an API description in the future, it's worth looking at tools which can use an open API description and use that to automate endpoint testing.