A 72-year-old woman with ischaemic cardiomyopathy (LVEF 30%), NYHA class II symptoms on ACE inhibitor and beta blocker presents for optimisation. She is euvolaemic with blood pressure 110/68 mmHg and eGFR 65 mL/min/1.73 m², potassium 4.6 mmol/L. Which additional therapy has the best evidence for reducing mortality?