Challenge 2 / 2

Does your User class need to be a Doctrine entity?

Technically no, but you'll need to create your own authenticator system to support a non-entity User class.

No: you're free to make your User class a Doctrine entity or not.

Yes. But it's no problem: if you load users data from somewhere else (e.g. an API), just store a local "cache" of that data in your local database.