Returns the alert-threshold override configuration that controls certificate-expiry alert severity levels. This is a sub-resource of cert-inventory; GET /cert-manager/cert-inventory returns the read-only aggregated inventory with effective thresholds already applied per certificate, while this endpoint exposes the editable override configuration itself. The response always includes global thresholds (built-in defaults 15/10/5 unless a global override has been set). byCertId and byCategory are only returned when overrides have been configured. Returns an error when the certificate inventory feature is not enabled on this cluster.