A 52-year-old man from rural Maharashtra presents with a 3-week history of progressive jaundice, pale stools, and dark urine. He denies fever, abdominal pain, or recent weight loss. On examination, he is afebrile, icterus is present, and the abdomen is soft with no hepatomegaly or splenomegaly. Laboratory investigations show: total bilirubin 8.2 mg/dL (conjugated 6.8 mg/dL), ALT 45 U/L, AST 52 U/L, ALP 320 U/L, GGT 280 U/L. Ultrasound abdomen reveals dilated intrahepatic and extrahepatic bile ducts with a hypoechoic lesion at the pancreatic head. What is the most likely diagnosis?