A 28-year-old man is brought to the trauma centre 2 hours after a stab wound to the left anterior chest (5th intercostal space, midaxillary line). He is haemodynamically stable (BP 128/82 mmHg, HR 94/min), with normal breath sounds bilaterally and no external bleeding. On examination, there is a small puncture wound with minimal surrounding erythema. Bedside FAST is negative. Which investigation is most appropriate to exclude occult haemothorax and pneumothorax?