A 28-year-old woman with a history of intravenous drug use presents with a 5-day history of left shoulder pain, swelling, and fever (38.2°C). Synovial fluid obtained by arthrocentesis shows WBC 62,000/μL (90% neutrophils), glucose 22 mg/dL (serum 110 mg/dL), protein 5.8 g/dL, and Gram stain is negative. Blood cultures are pending. She is allergic to penicillin (anaphylaxis). What is the most appropriate immediate antibiotic regimen?