A 52-year-old man from Delhi presents to the emergency department with a 3-day history of fever (39.5°C), productive cough with rust-coloured sputum, and pleuritic chest pain. On examination, he is tachypneic (RR 28/min), with crackles in the right lower lobe. Chest X-ray shows consolidation in the right lower lobe. Blood culture is pending. A Gram stain of sputum shows Gram-positive diplococci. Which of the following is the most appropriate initial antibiotic therapy for this patient?