A 78-year-old woman presents with palpitations and breathlessness. ECG shows atrial fibrillation with a ventricular rate of 120 beats/min. She has hypertension and heart failure with preserved ejection fraction but no valvular disease. Her CHA₂DS₂-VASc score is 4. She is currently on no anticoagulant therapy. Renal function is normal. What is the most appropriate long-term strategy to reduce her stroke risk?