A 55-year-old woman presents with a 6-week history of pain and stiffness in her shoulders and hips, which is worse in the mornings and improves as the day goes on. She reports difficulty getting out of bed and raising her arms above her head. She also feels generally unwell and fatigued. Her inflammatory markers are raised: ESR 65 mm/hr and CRP 50 mg/L. What is the most likely diagnosis?