A 28-year-old woman presents to the emergency department with acute lower abdominal pain and vaginal bleeding for 2 days. She reports a missed period 6 weeks ago. On examination, she is pale, tachycardic (HR 110/min), and has severe left-sided lower abdominal tenderness with guarding. Serum β-hCG is 8,500 mIU/mL. Transvaginal ultrasound shows an empty uterus with free fluid in the pouch of Douglas. What is the most appropriate immediate management?