A 32-year-old woman, 6 hours postpartum after vaginal delivery, develops sudden heavy vaginal bleeding (estimated 1200 mL in 30 minutes). She is pale, anxious, with HR 118/min, BP 92/58 mmHg, RR 22/min. Uterus is firm and well-contracted. Lochia is dark and clotty. What is the most appropriate immediate next step?