A 54-year-old man presents with 7 weeks of facial flushing and diarrhea. His symptoms began intermittently but are becoming more constant. A 24-hour urine collection reveals an elevated level of 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin. An abdominal CT scan shows a 2-cm mesenteric mass in the ileum and likely metastatic tumors in the liver. What is the likely diagnosis?