Function resolve_resend_target 

async fn resolve_resend_target(
    conn: &mut PgConnection,
    course_id: Uuid,
    payload: &ResendLinkingEmailPayload,
) -> Result<Option<String>, ControllerError>

The person the body names, as a student number. None when the account has never held one.

A user_id is only answered for an account with a registration on course_id. The caller is authorized on the course and the study registry roster is only consulted later, so without this one course’s teacher could hand in any account’s uuid and read off, from which outcome came back, whether that account holds a verified student number.