A 58-year-old man from rural Maharashtra presents with progressive dyspnea on exertion for 3 months and orthopnea for 1 week. He has a history of acute rheumatic fever at age 12. On examination, blood pressure is 140/90 mmHg, pulse 88/min regular, JVP elevated at 6 cm. Auscultation reveals a loud S1, an opening snap 80 ms after A2, and a mid-diastolic murmur best heard at the apex in the left lateral decubitus position. Chest X-ray shows pulmonary edema with straightening of the left heart border. ECG shows atrial fibrillation with a ventricular rate of 92/min and left atrial enlargement. What is the most likely diagnosis?