A 52-year-old man from Delhi presents with a 3-month history of progressive abdominal distension, early satiety, and weight loss of 8 kg. On examination, he has hepatomegaly (3 cm below costal margin), ascites, and a firm left supraclavicular lymph node. Ultrasound abdomen shows a heterogeneous mass in the gastric fundus with ascites. Serum AFP is 450 IU/mL (normal <20), and CEA is 8 ng/mL (normal <5). Upper endoscopy with biopsy confirms gastric adenocarcinoma. Which tumor marker is most useful for monitoring treatment response and detecting recurrence in this patient?