A 68-year-old man with diabetes and hypertension presents with a 6-hour history of severe right upper quadrant pain radiating to the right shoulder, accompanied by nausea and vomiting. He denies fever. On examination, he is afebrile (36.8°C), with severe RUQ tenderness and a positive Murphy's sign. Serum WBC is 18,500/μL, bilirubin 1.2 mg/dL, ALT 95 U/L, ALP 105 U/L, and amylase 70 U/L. Ultrasound shows a thickened gallbladder wall (5 mm), multiple echogenic foci consistent with stones, and no pericholecystic fluid. What is the most appropriate next step in management?