A 28-year-old man presents to the clinic with an 18-month history of insidious-onset low back pain and stiffness. He reports that the pain is worse in the morning and after periods of inactivity, and improves with exercise and activity. He also notes occasional pain around his heels. On examination, he has reduced lumbar flexion and chest expansion. There is tenderness at the Achilles tendon insertion site. MRI of the sacroiliac joints reveals bilateral sacroiliitis. Laboratory studies show a positive HLA-B27 and elevated CRP. Which of the following is the most appropriate initial treatment for this patient?