A 62-year-old man presents to the emergency department with coughing of bright red blood estimated at 300 mL over the past hour. He is tachypnoeic and anxious. Vital signs: blood pressure 140/80 mmHg, pulse 110 beats/min, oxygen saturation 90% on room air. Chest examination reveals unilateral coarse crackles. What is the most important immediate management priority?