A 30-year-old Afro-Caribbean man presents with recurrent episodes of severe limb and abdominal pain. He reports feeling tired and has noticed yellowing of his eyes. Blood tests show: Hb 78 g/L, Reticulocytes 15%, Bilirubin 55 µmol/L (conjugated 10 µmol/L). Blood film shows sickle cells and target cells. What is the underlying diagnosis?