A 6-year-old girl with known idiopathic generalized epilepsy is brought to the pediatric emergency department in status epilepticus. She received IV lorazepam 4 mg 10 minutes ago, but seizures have not stopped. Her vital signs are: HR 156/min, RR 32/min, BP 98/60 mmHg, SpO₂ 91% on room air. She is still actively convulsing with generalized tonic-clonic movements. What is the most appropriate next step in management?