A 70-year-old woman with long-standing hypertension and prior lacunar stroke presents with increasing dyspnoea on exertion, orthopnoea and bilateral ankle oedema. Echocardiography shows LVEF 30%. She is on an ACE inhibitor and a beta blocker at target doses. According to CCS heart failure guidance, which additional medication has the STRONGEST mortality benefit and should be added next if tolerated?