A 42-year-old woman undergoes elective cesarean section under spinal anesthesia. The procedure is uneventful, and she is discharged on postoperative day 2. On day 5, she develops a positional headache (worse when sitting/standing, better when lying flat), accompanied by neck stiffness and auditory symptoms (tinnitus). Vital signs are stable. What is the most likely diagnosis and the recommended initial management?