A 58-year-old man from rural India presents with progressive dyspnea on exertion for 6 months and orthopnea. He has a history of acute rheumatic fever (ARF) at age 12. On examination, he has an irregular pulse (rate 110/min), blood pressure 140/90 mmHg, and a loud S1 with an opening snap. Auscultation reveals a mid-diastolic murmur best heard at the apex in the left lateral decubitus position. Chest X-ray shows pulmonary congestion and straightening of the left heart border. Echocardiography confirms a valve area of 1.2 cm² with restricted leaflet motion and chordal shortening. What is the most likely diagnosis?