A 68-year-old woman with a 40-year smoking history presents with chronic productive cough and exertional dyspnoea. Spirometry confirms FEV1/FVC 0.55 and FEV1 55% predicted. She is not on inhaled therapy and has had two exacerbations in the last year treated with oral steroids. According to CTS guidelines, which maintenance pharmacologic therapy is most appropriate to start?