A 72-year-old woman with a 15-year history of heart failure with reduced ejection fraction (EF 28%) is on lisinopril, metoprolol succinate, and spironolactone. Her recent echocardiogram shows no improvement in ejection fraction despite 6 months of optimal medical therapy. She remains symptomatic with NYHA Class III dyspnea. Her current medications are well-tolerated; serum creatinine is 1.2 mg/dL and potassium is 4.8 mEq/L. Her blood pressure is 110/70 mmHg and heart rate is 58/min. What is the most appropriate next addition to her regimen?