A 28-year-old woman presents to the emergency department with 8 weeks amenorrhea, acute right lower abdominal pain, and vaginal spotting. Vital signs: BP 100/60 mmHg, HR 110/min, RR 20/min. Urine pregnancy test is positive. Transvaginal ultrasound shows an empty uterus, free fluid in the pelvis, and a 3.5 cm mass in the right adnexa with fetal heart activity. What is the most appropriate immediate next step in management?