A 32-year-old multiparous woman (G4P3) undergoes emergency lower-segment cesarean section at 38 weeks for placenta previa. Intraoperatively, the placenta is noted to be densely adherent to the lower uterine segment with invasion into the myometrium. After delivery of the baby and placenta, brisk vaginal bleeding continues despite oxytocin infusion and uterine massage. Blood loss at 30 minutes post-delivery is estimated at 1200 mL. Vital signs: BP 92/58 mmHg, HR 128/min, RR 24/min. Hemoglobin is 7.8 g/dL. What is the most appropriate next intervention?