feat(nip77): negentropy reconciliation core for the PostgreSQL backend #801

Is your feature request related to a problem? Please describe.
Clients and relay-to-relay syncers catching up with nostream currently send wide REQ filters, so the relay streams every matching event even when the other side already has most of them. NIP-77 (Negentropy) solves this with set reconciliation: both sides exchange range fingerprints and only transfer the events that actually differ. nostream has no NIP-77 support.

The strfry backend being added separately already implements negentropy itself, so this work targets the PostgreSQL backend only and must stay cleanly separable from it.

Describe the solution you'd like
This issue covers only the first, protocol-independent piece of NIP-77, so it can be reviewed on its own before any wire-level work:

• A negentropy reconciliation core: encode/decode of negentropy messages and fingerprint computation over a sorted set of (created_at, id) items, with no networking or WebSocket code.
• A new EventRepository method returning just (created_at, event id) for events matching a filter, sorted as negentropy requires. It reuses the shared filter conditions (so NIP-56 hidden reports are never advertised) and also excludes deleted and expired events, so they are never advertised either.
• Unit tests for the codec/fingerprint (including interop vectors from the reference implementation) and for the new query.

Follow-ups, tracked separately and not part of this issue: the NEG-OPEN/NEG-MSG/NEG-CLOSE/NEG-ERR wire protocol with per-connection sessions and limits, then settings, NIP-11 advertisement and backend gating.

Additional context

• Spec: https://github.com/nostr-protocol/nips/blob/master/77.md
• Reference implementation: https://github.com/hoytech/negentropy