HMAC-authenticated, rate-limited (60/min/peer) endpoint that resolves a
homeUserId on this instance to its canonical username + profile snapshot.
Native non-deleted users only. Mirrors /users/lookup's auth shape.
Adds lookupRemoteUserByHomeId to federationLookup.ts as the client-side
helper. Used by the upcoming stub-backfill worker on peers that hold legacy
snowflake-named replicas of users now visible by their real handle.
