feat(dashboard): live anomaly badge — background poll every 30s

[surpradhan]

What does this PR do?

Keeps the anomaly badge and Analyze nav dot current without requiring the user to visit the Anomalies sub-tab. A 30-second background poll fetches the anomaly count after page load and updates the UI silently.

Changes

src/public/dashboard.html only — no backend changes

• ANOMALY_POLL_MS = 30_000 constant
• refreshAnomalyBadge() — fetches GET /analytics/anomalies (with current threshold if set); if the user is already on the Anomalies tab it calls renderAnomalies() for a full refresh, otherwise updates only the badge count and pdot-analyze dot
• startAnomalyPoll() — clears any prior interval, fires an immediate fetch, then schedules repeats
• Wired into loadAll() after first successful data fetch (authenticated path only)
• Errors are silenced — badge stays stale rather than flashing on transient network issues

How to test

npm run ingest
# open dashboard — Anomalies badge starts at 0 (or current count)
# emit events that trigger anomalies; without switching tabs, observe
# the badge and Analyze dot update within 30s

Checklist

• Tests pass (243/243, no new tests needed — pure UI behaviour)
• Lint clean
• No backend changes, no new CI jobs, no schema migrations

Part of UX polish sprint (PR-D of 4, stacked on PR-C)

[surpradhan]

R1 findings (all addressed in 21ce12e): (1) poll kept running after 401 → now clears interval on Unauthorized; (2) badge queried different range than panel → now reads shared-since/shared-until via toIso(); (3) poll fired while tab hidden → early return on document.hidden; (4) duplicate banner line + bare fire-and-forget call → fixed (void keyword). R2: both reviewers approve, zero actionable items. 243/243, lint clean. Approve.