A 32-year-old man presents to the emergency department with acute onset severe right lower abdominal pain, fever (38.5°C), and localized tenderness over McBurney's point. Abdominal ultrasound shows a dilated, non-compressible appendix (8 mm diameter) with surrounding free fluid. WBC is 14,200/μL with left shift. What is the most appropriate immediate next step in management?