Previously, we had it session-scoped, which meant that the app was
created once for all tests. This doesn't allow us to play with different
settings however, such as disabling account registration or image
uploads and testing that.

Now, the fixtures are module scoped, which means that we still save on
prepare/teardown time a bit, but we're also a bit more flexible. By
making new test modules, we can test different settings there.

The most "painful" one was Pydantic V2, not because it didn't work, but
because I wanted to do it right and not use the deprecated methods.

See if it works, it might be handy for other tests in the future as
well.