A 52-year-old man from Delhi presents with a 10-month history of progressive proximal muscle weakness, easy bruising, and recurrent infections. On examination, he has marked central obesity with supraclavicular fat pads, a dorsocervical fat pad ('buffalo hump'), and multiple purple striae on his abdomen and thighs. Blood pressure is 162/104 mmHg. Investigations reveal: 24-hour urinary free cortisol 420 µg/day (normal <50), morning plasma cortisol 38 µg/dL (normal 5–25), ACTH 68 pg/mL (normal 10–50). Low-dose dexamethasone suppression test (1 mg overnight) shows cortisol of 32 µg/dL. High-dose dexamethasone suppression test (8 mg overnight) shows cortisol of 8 µg/dL. What is the most likely diagnosis?