A 35-year-old woman from Kolkata presents with a 7-day history of high fever, headache, and abdominal pain. Blood culture grows Salmonella typhi. She is started on ceftriaxone 2 g IV 12-hourly. On day 5 of therapy, she develops severe diarrhea with bloody stools and signs of peritonitis (rigid abdomen, rebound tenderness). Abdominal imaging shows free fluid. What is the most appropriate next step in management?