A 58-year-old man from Delhi presents to the emergency department with acute onset dyspnea and pleuritic chest pain for 6 hours. He has a history of recent orthopedic surgery 10 days ago. Vital signs: HR 110/min, BP 128/82 mmHg, RR 24/min, SpO₂ 92% on room air. D-dimer is elevated at 2.5 μg/mL (normal <0.5). Chest X-ray is normal. CT pulmonary angiography (CTPA) shows a subsegmental pulmonary embolism in the right lower lobe with no signs of right ventricular strain. What is the most appropriate next step in management?