insights: Start investigating adding backend API tests

Created by: CristinaBirkel

This is a spike issue to investigate what it will take to add tests for our API and get a feel for the scope here. These tests will be crucial for us to confidently refactor, fix bugs and make small feature changes. There is a good deal of refactoring/deprecation work to clean up the code and I think it makes sense to have these tests first.

Note: I'm going to estimate it at 1 day for now, to keep in line with our "10% tech debt" rule, but I would like to estimate it at 2 days otherwise. So the outcome of this issue may be some initial findings and another spike issue to keep investigating.