A 72-year-old woman from Kolkata with a history of diverticulosis presents with acute left lower abdominal pain, fever (39.1°C), and signs of peritonitis. Laboratory studies show WBC 16,000/μL and elevated CRP. CT abdomen with IV contrast reveals free air in the peritoneal cavity, a focal defect in the sigmoid colon wall, and localized purulent collection measuring 4 cm adjacent to the perforation site. What is the most appropriate next step in management based on these imaging findings?