A 50-year-old lifelong non-smoker presents with a chronic cough productive of copious amounts of purulent sputum (about half a cupful each morning) for the past two years. He reports recurrent episodes of chest infections requiring antibiotics, approximately 3-4 times per year. A chest X-ray shows prominent bronchial wall thickening and some 'tram-track' lines, particularly in the lower lobes. What is the most likely underlying diagnosis?