A 32-year-old multiparous woman (G3P2) undergoes emergency cesarean section at 38 weeks for placenta previa. Intraoperatively, the lower uterine segment is thin and friable. The placenta is removed with difficulty, and the surgeon notes a small area of myometrial invasion at the placental bed. Estimated blood loss is 1200 mL. In the recovery room, 2 hours postoperatively, the patient has continuous vaginal oozing. Vitals: BP 95/58 mmHg, HR 118/min, RR 24/min. Hemoglobin is 8.5 g/dL (down from 10.8 g/dL preoperatively). Uterus is firm on palpation. What is the most likely diagnosis?