A 68-year-old woman with chronic atrial fibrillation and a history of rheumatic mitral stenosis presents with acute dyspnea and orthopnea. On examination, she is tachypneic (RR 28/min), has a loud opening snap followed by a long diastolic murmur, and bilateral crackles on auscultation. Chest X-ray shows pulmonary edema. Echocardiography confirms severe mitral stenosis (mitral valve area 0.8 cm²) with rapid ventricular response (HR 140/min). What is the most appropriate immediate next step in management?