A 38-year-old woman with rheumatoid arthritis presents to the acute pain clinic with severe left knee pain (8/10) following intra-articular corticosteroid injection 48 hours ago. She reports acute swelling, warmth, and erythema over the injection site. Temperature is 37.8°C, and synovial fluid aspirate shows 45,000 WBC/μL with 80% neutrophils. Gram stain and culture are pending. What is the most appropriate immediate management?