Add a push/scrape surface for the event ledger

[JSONbored]

Problem: The event ledger is a genuine structured audit trail, but it's pull-only via ledger list --json — there's no push/webhook/scrape surface for a self-hoster's own Grafana or alerting to ingest it.

Area: AMS / Observability

Proposal: Add an optional webhook-push or Prometheus-scrape-compatible surface for new event-ledger entries.

Deliverables:

• One new consumption path for the event ledger beyond ledger list --json.

Acceptance criteria:

• A new event-ledger entry is observable via the new surface without polling ledger list.

[linear-code]

JSO-3501