A 32-year-old man presents with pleuritic chest pain that worsens with deep inspiration and lying supine and improves when he sits forward. He had a viral upper respiratory infection one week ago. Temperature is 37.6 °C, blood pressure 118/70 mmHg, heart rate 96 beats/min. ECG shows diffuse concave ST-segment elevation with PR depression. Echocardiogram reveals a small pericardial effusion without tamponade. What is the most appropriate initial treatment?