Function list_unlinked_enrolled_before 

pub async fn list_unlinked_enrolled_before(
    ctx: &ManualActionContext<'_>,
    course_code: &str,
    since: DateTime<Utc>,
) -> CreditRegistrationResult<Option<Vec<RosterPerson>>>

Lists one course code’s roster now and keeps the people linking mails skip: enrolled before since or at no known time, and linked to no account, each once. Stores nothing.

Ok(None) means the registry gave no usable answer.