A 28-year-old woman is 2 hours postpartum after a vaginal birth. She suddenly develops heavy vaginal bleeding with clots. Fundus is soft and above the umbilicus; estimated blood loss is 1200 mL, BP 90/60 mm Hg and HR 120 beats/min. According to SOGC postpartum haemorrhage guidance, what is the most appropriate immediate first-line intervention?