v2.82.3 — Board honesty fixes (found by admin bughunt)

Board honesty fixes (found by admin bughunt)

• Cost savings no longer fabricated. The board's metrics recomputed their own
  human-cost estimate (done × 4h × $150) and savings multiplier, ignoring the
  backend — which deliberately returns savings_x: null when there's no measured
  verdict cost data. That produced an absurd "5,538×" out of nothing. The
  frontend now uses the backend's cost.savings_x / cost.human_usd; with no
  measured data it shows "—", not a made-up multiplier (a real number still
  renders when verdict cost data exists).
• In-app notifications dedupe by key. The alert crons tick every 5 min and a
  persistent condition (open P0, blocked/stale gate, cost over threshold, and the
  daily digest across its whole UTC hour) added a fresh bell notification on every
  tick — email/push already deduped, the in-app one didn't. addNotification now
  takes a dedupeKey and skips duplicates; all crons pass theirs.
• Update-check cache freshness is inclusive at the exact 24h boundary
  (< → <=) — no needless background refresh on the tick.
• Tests: board suite 56 → 60 (+4 notification-dedup regression tests via a new
  GREAT_CTO_NOTIF_HISTORY_FILE test seam); CLI update-check suite green.

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