A 52-year-old woman undergoes tracheostomy for long-term mechanical ventilation following a severe stroke with bulbar dysfunction. On postoperative day 5, she develops sudden onset of subcutaneous emphysema in the neck and chest wall, with audible air leakage around the tracheostomy tube. Her oxygen saturation drops to 88% on FiO₂ 0.5, and she has mild stridor. Chest X-ray shows pneumomediastinum and subcutaneous air tracking into the chest. What is the most appropriate immediate management?