A 38-year-old woman from Mumbai with a history of acute rheumatic fever at age 8 presents with progressive dyspnea and palpitations over 2 months. On examination, she has an opening snap and a low-pitched diastolic rumble at the apex, with an irregular pulse (rate 110/min). ECG shows atrial fibrillation. Transthoracic echocardiography reveals mitral stenosis with a valve area of 1.2 cm² and moderate left atrial enlargement. Left ventricular ejection fraction is 55%. There is no evidence of thrombus on transesophageal echocardiography. What is the most appropriate next step in management?