[Admin Console] : trust & automation (audit, webhooks, OTel, diagnostics) #919

Goal

Ship operator trust features: auditable admin actions, automated alerts, probe/mirror metrics, and a safe support export.

Scope

1. Audit log — append-only record of admin login/logout, settings changes, manual NIP-66 probe, notification/webhook config changes; GET /admin/audit-log and read-only UI.
2. Outbound webhooks — configurable URL(s) and secret; events: probe.failed, probe.recovered, ssl.expiring, mirror.disconnected (signed payload, retries). Extend operator notifications where it fits.
3. OpenTelemetry — probe & static-mirror metrics (success/failure, latency, reconnects) consistent with existing OTel/metrics setup.
4. Diagnostics export — download/endpoint with version, redacted settings, recent health and worker/mirror summary (no secrets).

Closes #688 .