Function claim_unnotified 

pub async fn claim_unnotified(
    conn: &mut PgConnection,
    scope: &RegistrationScope,
    limit: i64,
) -> ModelResult<Vec<StudentNotificationToQueue>>

Claims the rows owed a mail, locking them until the caller’s transaction ends, so callers must pass a transaction. Never claims cancelled, blocked or any failure state: those get nothing. Nor a duplicate or not_improved row whose student already has, or is owed, the registered mail for another row of the module.