A 48-year-old woman from Mumbai undergoes wide local excision (WLE) for a 1.8 cm invasive lobular carcinoma (ILC) of the left breast. Final histopathology shows clear margins (>5 mm), no lymphovascular invasion, and sentinel lymph node biopsy is negative (0/2 nodes). Tumour is ER/PR positive, HER2 negative. She is anxious about recurrence. What is the most appropriate next step in management?