Refactors the POST handler into handleLocalFriendRequest + handleFederatedFriendRequest helpers. The federated branch resolves the target domain, ensures peering, looks up the remote user, creates/hydrates a replicated stub, and writes a transactional (friend_requests + mutation_log + outbox) event with relayMessageId set. Also exports hydrateReplicatedUserProfile from federation.ts and adds the T11 happy-path test.