A 55-year-old female with a history of well-controlled depression on sertraline presents reporting a persistent sensation of "ants crawling" on her legs at rest in the evenings. The sensation is relieved by walking or moving her legs. It significantly disrupts her ability to fall asleep. She reports this has been present for the past 6 months. Her ferritin level is 18 ng/mL (low). Physical and neurological exam are otherwise unremarkable. What is the MOST likely diagnosis?