A 38-year-old woman presents with a 3-week history of purulent nasal discharge, facial pain over the cheeks and forehead, and post-nasal drip. She reports low-grade fever (37.8°C) and denies recent upper respiratory infection. On examination, the nasal mucosa is edematous and pale, and palpation over the maxillary sinuses elicits tenderness. A non-contrast CT scan of the paranasal sinuses shows opacification of the maxillary and ethmoid sinuses with air-fluid levels. What is the most likely diagnosis?