A 22-year-old male presents to the emergency department with a 36-hour history of periumbilical pain that migrated to the right lower quadrant. He reports anorexia, nausea, and one episode of vomiting. On examination, he is febrile (38.5°C), and there is marked tenderness at McBurney's point with guarding and rebound tenderness. Rovsing's sign is positive. His WBC is 13,500/μL with left shift. Abdominal ultrasound shows a non-compressible, dilated appendix (8 mm) with surrounding free fluid. What is the most appropriate next step in management?