A 22-year-old woman presents to the emergency department with acute shortness of breath and pleuritic chest pain. She started the combined oral contraceptive pill 6 months ago. She returned from a 20-hour flight from Europe 3 days ago. Vital signs: BP 110/70 mmHg, pulse 118 bpm, RR 24 breaths/min, SpO₂ 93% on room air, temperature 37.2 °C. You calculate a high pre-test probability of pulmonary embolism. What is the most appropriate initial imaging test?