A 6-year-old girl presents with a 5-day history of left ear pain, purulent discharge from the ear canal, and fever (38.5°C). Her parents report she had a cold 2 weeks prior. On otoscopy, the tympanum shows a small perforation with pus draining from the middle ear. Audiometry shows a 25 dB conductive hearing loss. What is the most likely organism and the recommended first-line antibiotic?