A 28-year-old woman is brought to the emergency department by her boyfriend after she threatened self-harm during an argument. She has a 6-year history of unstable relationships, impulsive spending, binge-eating episodes, and recurrent suicidal gestures. She reports intense fear of abandonment and describes her relationships as alternating between idealization and devaluation. She denies psychotic symptoms. Vital signs are stable, and she is cooperative. What is the most appropriate immediate next step?