fix: exclude archived cases from stale-case-reminder

Archived cases have archived_at IS NOT NULL — they are not "stuck",
they are done. The stale query was missing this filter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/app.py

@@ -1148,6 +1148,7 @@ async def api_stale_cases(days: int = 3):
                    EXTRACT(DAY FROM (now() - updated_at))::int AS days_stale
             FROM cases
             WHERE status NOT IN ('final', 'new', 'exported')
+              AND archived_at IS NULL
               AND updated_at < now() - make_interval(days => $1)
             ORDER BY updated_at ASC  -- oldest stale first (longest overdue = highest priority)
             """,