A 38-year-old man with a 10-year history of alcohol abuse presents to the emergency department with severe epigastric pain radiating to the back, nausea, and vomiting for 6 hours. On examination, he is febrile (38.5°C), tachycardic (110/min), and has severe epigastric tenderness with guarding. Serum amylase is 1200 U/L (normal <100) and lipase is 2400 U/L. Abdominal ultrasound shows a swollen pancreas with peripancreatic fluid. What is the most appropriate immediate next step in management?