A 28-year-old woman presents with episodic wheeze and dyspnoea. Spirometry shows FEV1 70% predicted with a 15% and 250 mL improvement after inhaled salbutamol. She has symptoms twice weekly and uses a short-acting beta-agonist inhaler accordingly. According to the Canadian Thoracic Society asthma guideline, what is the preferred controller strategy for mild asthma?