A 68-year-old woman with colorectal cancer presents with acute abdominal pain, distension, and vomiting. Plain abdominal X-ray shows dilated small bowel loops with an air-fluid level and free air in the peritoneal cavity. Contrast-enhanced CT confirms a perforated small bowel loop proximal to a large obstructing mass in the sigmoid colon. Her vital signs are stable (BP 110/70 mmHg, HR 95/min, RR 20/min, Temperature 37.5°C). What is the most appropriate next step in management?