A 28-year-old woman from rural Maharashtra presents to the gynaecology outpatient department with vaginal bleeding and lower abdominal pain for 3 weeks. She reports amenorrhoea for 2 months. On examination, the uterus is enlarged disproportionately (size consistent with 16 weeks gestation) but soft. Serum β-hCG is markedly elevated at 450,000 mIU/mL. Transvaginal ultrasound shows a heterogeneous echogenic mass filling the uterine cavity with multiple small cystic spaces ('bunch of grapes' appearance) and no fetal parts. There is no evidence of theca lutein cysts. What is the most likely diagnosis?