A 52-year-old woman with treatment-resistant depression presents to the outpatient clinic. She reports persistent depressed mood, anhedonia, and increasing preoccupation with death over the past 6 weeks. She denies active suicidal ideation or intent but admits she 'wouldn't mind if something happened.' She lives alone, is recently divorced, and has access to her late husband's firearms stored in her home. She has failed two adequate trials of SSRIs. What is the most appropriate immediate next step?