feat(admin): bounded NIP-66 probe history and Network Health timeline #800

Parent

Part of #688

Context

• Latest probe snapshot + GET /admin/network-health and dashboard panel exist (feat(nip66): add shared relay probe engine (DNS, TLS, WS RTT, NIP-11) #669).
• NIP-11 vs probe mismatch warnings exist ([Admin Console] NIP-11 vs probe mismatch warnings on Network Health #798).
• Today only nip66:snapshot:latest is stored (Redis); there is no history API or timeline UI.

Goal

Give operators bounded probe history and a dashboard timeline so they can see trends (flapping TLS, NIP-11 failures, RTT drift), not only the last run.

Scope

• Persist bounded probe run history.
• Will reuse existing snapshot shape.
• GET /admin/network-health/history paginated or ?limit= (auth + IP whitelist same as other admin routes).
• nip66 retention setting (e.g. historyRetentionRuns or historyRetentionHours) in default-settings.yaml, validation and CONFIGURATION.md.
• Dashboard: timeline / last-N runs.
• Unit/integration tests for store + controller; update admin route tests.