feat(nip66): add relay monitor settings schema and defaults #687

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)