Look into using Django Ninja test client instead of live_server

[mrchrisadams]

At present we use the pytest django live_server fixture when testing some API responses in the test_api_external.py file. these work, but using the entire live_server is slow.

There is a way to use a special router in Django Ninja - this would allow us to use the more lightweight Django test client, giving us faster tests, but involves a bit of work to get pytest and django ninja to work together - maybe even introducing a Router class.

It's not urgent, would be useful to have implemented.

https://django-ninja.dev/guides/testing/