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 tenderness is elicited at McBurney's point with positive Rovsing's sign and rebound tenderness. His WBC is 13,500/μL with left shift. Ultrasound shows a non-compressible appendix measuring 8 mm with surrounding free fluid. What is the most appropriate next step in management?