A 28-year-old woman from Mumbai with a history of systemic lupus erythematosus (SLE) on hydroxychloroquine presents with acute onset of left ankle pain, swelling, and warmth for 3 days. She denies trauma. Temperature is 38.8°C, and the ankle is visibly swollen with restricted dorsiflexion. Ankle aspiration yields cloudy fluid: WBC 45,000/μL (88% neutrophils), glucose 22 mg/dL (serum 110 mg/dL), protein 4.8 g/dL. Gram stain is negative. Blood cultures are sent. What is the most likely causative organism, and what is the recommended empiric antibiotic regimen?