Function mark_pending_superseded 

pub async fn mark_pending_superseded(
    conn: &mut PgConnection,
    id: Uuid,
    superseded_by_id: Uuid,
) -> ModelResult<()>

Marks a registered row as being replaced by superseded_by_id, a later attempt with a better grade, of the same completion or another, which takes over the row’s slot in uq_credit_registrations_person_module.

Not mark_superseded: the row stays the live credit until the new attempt is registered, since Sisu may accept the better grade without ever making it the course unit’s attainment. transition then supersedes the row, or clears the mark if the new attempt stops short.