A 45-year-old woman with a history of iron deficiency anemia is admitted for a blood transfusion due to symptomatic anemia. About 30 minutes after receiving a unit of packed red blood cells, she develops fever 38.5°C, chills, and mild discomfort, but no rash, hypotension, or dyspnea. Her vitals remain stable, and physical examination does not reveal signs of an allergic reaction or hemolysis. Laboratory tests show: Hemoglobin: 9.8, WBC: 7,500/mm³, Platelet count: 250,000/mm³, LDH: Elevated, Haptoglobin: Low. Direct Coombs test: Positive. Urinalysis: Hemoglobinuria (no RBCs on microscopy). What is the most likely diagnosis?