A 68-year-old man with NYHA class II heart failure with reduced ejection fraction (LVEF 32%) is on maximally tolerated ACE inhibitor, beta blocker, mineralocorticoid receptor antagonist and diuretic. He remains symptomatic and has type 2 diabetes with eGFR 55 mL/min/1.73 m². According to CCS heart failure guidelines, which additional medication class should be added to reduce hospitalisations and mortality?