A 38-year-old woman from Mumbai with a 10-year history of chronic pancreatitis (secondary to alcohol) presents with acute-on-chronic epigastric pain, fever (38.8°C), and elevated inflammatory markers (WBC 14,500/μL, CRP 120 mg/L). Serum amylase is 650 U/L and lipase 950 U/L. Contrast-enhanced CT abdomen shows a 4 cm pseudocyst with thick, enhancing wall and internal debris, located in the pancreatic body. There is no peripancreatic necrosis. Blood cultures grow Escherichia coli. Despite 48 hours of broad-spectrum antibiotics and supportive care, she remains febrile with persistent pain. What is the most appropriate next step in management?