A 52-year-old woman with a 10-year history of cholelithiasis presents to the emergency department with acute onset right upper quadrant pain, fever (38.5°C), and a positive Murphy's sign. Ultrasound shows a thickened gallbladder wall (5 mm), pericholecystic fluid, and impacted stone in the Hartmann's pouch. Laboratory values: WBC 14,000/μL, bilirubin 1.2 mg/dL, AST 45 U/L. What is the most appropriate immediate next step in management?