A 45-year-old woman with no prior abdominal surgery presents to the emergency department with acute onset severe epigastric pain radiating to the back, followed by sudden onset of severe diffuse abdominal pain and rigidity. On examination, she is in severe distress with board-like abdominal rigidity and rebound tenderness. Upright chest radiograph shows free air under the diaphragm. Serum lipase is elevated at 450 U/L. What is the most appropriate next step in management?