A 28-year-old man from Delhi presents to the emergency department with a 2-day history of high fever (39.5°C), severe frontal headache, neck stiffness, and photophobia. On examination, he is alert but irritable. Kernig's sign and Brudzinski's sign are both positive. Blood culture is pending. CSF analysis shows: protein 280 mg/dL, glucose 35 mg/dL (serum glucose 120 mg/dL), WBC 450/μL (90% neutrophils), Gram stain shows gram-positive cocci in pairs. Which is the most appropriate empirical antibiotic regimen for this patient?