Peering: sync with configured peers via NIP-77 reconciliation and a live feed

Part of the relay peering work in #931.

Scope

Wire the transfer of events between configured, authenticated peers using existing primitives — no new wire protocol:

On establishing a peering relationship, reconcile history with a NIP-77 negentropy session scoped to the agreed feed scope, so a new or restarted peer catches up without a full dump.
Stay current with a live subscription for the feed scope.
Build the peer sync path on NIP-77 directly: the StaticMirroringWorker was removed in chore: sunset the StaticMirroringWorker #924, so there is no existing mirroring machinery to reuse. The authenticated peer identity (Peering: authenticate peer relays with NIP-42 using the relay's own keypair #933) and the terms/scope from the advertisement (Peering: advertise peering terms and scopes in the NIP-11 document #932) take the place of static mirroring's config-file peer list — one sync design driven by the peering advertisement, not a second engine.

A peer that starts empty converges to the source's state within the agreed scope via negentropy.
Events published to the source after sync arrive at the peer live.
Sync state per peer (last reconciliation, lag) is recorded for the admin console.