Dedupe/downgrade users-list-overflow alert before it starts paging on every admin page load

src/lib/users-actions.ts:191-214 (MARS-615 follow-on): the users-list-overflow warn+email fires on EVERY listUsers() call once totalCount>USERS_HARD_LIMIT (500) — active count is already 756 and only grows, so this is now permanent steady-state, not a transient condition, and the appEvent write has no dedup (only the email is outbox-deduped same-day). Will page PM/Elazar repeatedly with zero new information per occurrence. Fix: dedupe the appEvent write the same way as the email (e.g. skip logging if an identical warn already fired in the last 24h), OR downgrade the appEvent to info severity since the page itself is healthy/paginated and this is no longer a failure signal. Preserve the actual business intent (notify when active-user capacity crosses a milestone) without per-pageview noise.