A 28-year-old woman from Bangalore with treatment-resistant schizophrenia has been on clozapine 400 mg/day for 8 months with good symptom control. She now presents with a 2-week history of progressive fatigue, dyspnea on exertion, and palpitations. On examination: HR 110/min, RR 24/min, BP 105/68 mmHg. Chest X-ray shows cardiomegaly and pulmonary edema. Echocardiography reveals dilated cardiomyopathy with ejection fraction 28%. What is the most likely diagnosis and the recommended next step?