feat: Redis nonce backend in standalone @bpc/server middleware (not just Ultra Server)

[rblake2320]

Summary

The RedisNonceStore is implemented and wired in Ultra Server (selfconnect-enterprise), but the standalone @bpc/server middleware does not expose a Redis nonce path directly. Consumers of @bpc/server who are not using Ultra Server have no documented path to Redis-backed nonce deduplication.

What is needed

• Export RedisNonceStore from @bpc/server as a first-class export (it may already be exported — verify)
• Add documentation showing how to wire RedisNonceStore into a standalone Express app using @bpc/server middleware
• Add an integration test in the bpc-protocol repo that uses RedisNonceStore directly

Effort estimate

~1 day (mostly documentation and test, implementation may already exist)