Function unlink_verified_student_number 

pub async fn unlink_verified_student_number(
    conn: &mut PgConnection,
    verified_student_number_id: Uuid,
    subject_user_id: Uuid,
    actor_user_id: Option<Uuid>,
    event_kind: CreditRegistrationEventKind,
    message: &str,
) -> ModelResult<i64>

Retires a verified link and reopens every registration it had gated, as one step. Both the student’s own unlink and the admin unlink call this rather than each doing the soft-delete and the recompute themselves; callers keep owning the transaction and any admin-action row around it.