A 38-year-old man with a history of gallstones presents to the emergency department with acute epigastric pain, nausea, and vomiting for 8 hours. Serum lipase is elevated at 1200 U/L (normal <100). Abdominal examination reveals epigastric tenderness without peritoneal signs. Contrast-enhanced CT abdomen shows pancreatic edema without necrosis, and no evidence of biliary obstruction. Vital signs are stable. What is the most appropriate immediate next step in management?