Reject existing-user claims in FastAPI signup flow

[PrivateBasicsApp]

Prevent unauthenticated account takeover where /signup/ could initialize UserInfo for a pre-provisioned user and immediately return a valid auth token.

Details:

• Preserve existing new-user creation logic and UserInfo initialization for truly new accounts.
• In signup_save (etebase_server/fastapi/routers/authentication.py) check for an existing username via get_user_queryset(...).get(...) and immediately raise HttpError("user_exists", ...) (HTTP 409) if found.
• Remove the previous behavior that only rejected the request when instance already had a userinfo, so the endpoint is now strictly for new-user creation.