A 42-year-old woman with seropositive rheumatoid arthritis (RF+, anti-CCP+) of 8 months' duration presents with persistent polyarticular swelling and morning stiffness of 2 hours despite 6 weeks of methotrexate monotherapy at 15 mg/week. ESR is 52 mm/h, CRP is 18 mg/L, and DAS28-CRP is 5.8. Plain radiographs of hands show early erosive changes. What is the most appropriate next step in management?