A 58-year-old man from Delhi presents with painless jaundice, pale stools, and dark urine for 3 weeks. Abdominal examination reveals a palpable gallbladder. Serum bilirubin is 8.2 mg/dL (direct 6.8 mg/dL), and CA 19-9 is elevated at 180 U/mL. Transabdominal ultrasound shows a hypoechoic lesion in the pancreatic head with dilated intrahepatic bile ducts and a dilated common bile duct (12 mm). What is the most appropriate next step in management?