Function mark_improvement_checked 

pub async fn mark_improvement_checked(
    conn: &mut PgConnection,
    id: Uuid,
    completion_updated_at: DateTime<Utc>,
) -> ModelResult<()>

Records that the grade-improvement scan looked at this accepted attempt against a completion in the given revision and found nothing better.

completion_updated_at must be the updated_at the scan actually read, not now(): the point is that the row stops being a candidate until the completion changes again.