A 28-year-old woman with a lifelong pattern of unstable relationships, intense fear of abandonment, chronic feelings of emptiness, and recurrent self-harm (cutting) presents to the emergency department after a heated argument with her boyfriend. She reports suicidal ideation with a vague plan. She has had 3 previous psychiatric admissions in the past 2 years, all for similar crises. Mental status examination shows dysphoria and mild agitation but no psychotic features. What is the most appropriate next step in management?