- Batch queries replace per-bill loops in tracker and analytics
- monthly_bill_state, payments, prev month payments batched with WHERE IN
- Empty billIds guards prevent SQL errors
- Hudson security audit: 5/5 PASS (SQL injection, empty IN, user scoping, data leakage, type safety)
- 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)