feat(nip66): add relay monitor settings schema and defaults

Scope: Add nip66.* to default-settings.yaml and settings types

Fields like:
- nip66.enabled (default false)
- nip66.probeIntervalSeconds
- nip66.timeouts (dns/tls/ws/nip11)
- nip66.targets (public relay URL(s) to probe)
- nip66.monitorPrivateKey / pubkey (or dedicated key path)
- nip66.dnsCacheTtlSeconds
- No worker yet, config only

Depends on: PR #675 (probe engine)