Peering: on-demand event requests between peers under peer limits #936

Part of the relay peering work in #931.

Scope

Beyond the standing feed and periodic reconciliation, let a peer fetch specific events on demand:

• A peer may send ordinary REQ or an ad-hoc negentropy session at any time — by event ID or filter — e.g. completing a thread whose root sits outside the feed's kinds/time range, or deep backfill, without widening the standing feed.
• Requests from authenticated peers are served under peer limits, not client limits — but they are still bounded: peer-specific request rate and response-size caps, so a peer credential is not an egress-abuse vector.
• Requests are covered by the flat peering terms. Serving a request never generates a per-request or per-event charge.
• Requests and the events they serve are accounted per peer for the admin console.

Acceptance

• A relay can fetch a specific out-of-scope event from a peer by ID without changing feed configuration.
• Requests beyond peer request limits are throttled; the same request pattern from a non-peer is treated as ordinary client traffic.