A 27-year-old woman presents with acute onset shortness of breath and pleuritic chest pain 2 days after a long-haul flight. She takes combined oral contraceptive pills. Vital signs: T 98.9°F, BP 118/70 mm Hg, HR 110 bpm, RR 22/min, SpO2 93% on room air. CT pulmonary angiography confirms a segmental pulmonary embolism. She is hemodynamically stable. Which is the most appropriate initial anticoagulation strategy?