Add a new user to the database.

This endpoint backs two flows that share the same route:

• Public sign up (unauthenticated). Allowed only when public registration is enabled for the deployment, i.e. AUTO_LOGIN is off (multi-user mode) and ENABLE_SIGNUP is True.
• Admin "add user" (authenticated active administrator). Always allowed, regardless of the sign up settings, so disabling public sign up does not break administrator-driven user creation.

User activation is controlled by the NEW_USER_IS_ACTIVE setting.