A 14-year-old female presents with a 2-month history of fatigue and symmetric joint pain involving her wrists, knees, and small joints of the hands. On examination, she has mild synovitis and an erythematous facial rash. Laboratory evaluation reveals a white blood cell count of 3,200/μL (normal 4,500-11,000/μL), positive antinuclear antibody at 1:320, and low C3 and C4. What is the most likely diagnosis?