A 28-year-old woman from rural Maharashtra presents to the gynecology outpatient department with vaginal bleeding and lower abdominal pain for 3 weeks. She reports a missed period 8 weeks ago. On examination, the uterus is enlarged to 14 weeks size, which is disproportionate to the reported period of amenorrhea. Serum β-hCG is markedly elevated at 450,000 mIU/mL. Transvaginal ultrasound shows a heterogeneous intrauterine mass with multiple small anechoic spaces ('bunch of grapes' appearance) and no fetal parts. There is no evidence of theca lutein cysts. What is the most likely diagnosis?