add UserAnswer model and endpoints

[lkeegan]

The questions that the user should answer about themselves are modelled in the database by UserQuestion:

https://github.com/ssciwr/mondey/blob/2daf2483221204a7d0c82fb0bd905846b5d5868e/mondey_backend/src/mondey_backend/models/questions.py#L30-L40

There is an endpoint to retrieve the list of questions here:

https://github.com/ssciwr/mondey/blob/2daf2483221204a7d0c82fb0bd905846b5d5868e/mondey_backend/src/mondey_backend/routers/questions.py#L15-L22

We need to add a UserAnswer model to store the answer by a particular user for a particular question, e.g. something like

UserAnswer

• id: int
• user_question_id: int
• user_id: int
• answer: str

And we need endpoints to retrieve the current set of answers, and to upload the new set of answers. I'd suggest adding these to the user router: https://github.com/ssciwr/mondey/blob/main/mondey_backend/src/mondey_backend/routers/users.py

We don't need full CRUD endpoints for these, just a get that returns the list of current answers for the user (if any), e.g.

@router.get("/user-answers/", response_model=list[UserAnswerPublic])
    def get_current_user_answers(session: SessionDep, current_active_user: CurrentActiveUserDep):
        ....

and a put that replaces the existing answers with new ones, e.g.

@router.put("/user-answers/")
    def update_current_user_answers(session: SessionDep, current_active_user: CurrentActiveUserDep, answers: list[UserAnswerPublic]):
        ....