A 52-year-old man from Mumbai presents with hypertension (165/105 mmHg), proximal muscle weakness, and easy bruising. Laboratory findings: K⁺ 3.1 mEq/L, glucose 156 mg/dL (fasting), 24-hour urinary free cortisol 285 µg/day (normal <50 µg/day), and 8 AM plasma ACTH 8 pg/mL (normal 10–50 pg/mL). Overnight dexamethasone suppression test (1 mg) shows cortisol 18 µg/dL (suppressed from baseline 28 µg/dL). High-dose dexamethasone (8 mg overnight) shows cortisol 4.2 µg/dL (suppressed >50% from baseline). What is the most likely diagnosis?