- Backend: previous month calculation with year wrapping (Jan→Dec)
- Backend: previous_month_paid per bill row, previous_month_total in summary
- Frontend: 'Last Month' column in desktop table with muted text
- Frontend: 'Last Month' in mobile view, summary card for prev month total
- Hudson security audit: 5/5 PASS (SQL injection, date wrapping, user scoping, auth, XSS)