feat: add multi-node horizontal scaling foundations

NotificationBus for cross-node message delivery fan-out:
- NotificationBus trait: publish(topic) + subscribe(topic) -> Notify
- InMemoryNotificationBus: single-node default via tokio::sync::Notify
- Designed for Redis pub/sub or NATS replacement in multi-node deploys
- 3 async tests: publish wakes, timeout without publish, independent topics

Health endpoint enhancements for load balancer awareness:
- HealthResponse proto: add node_id, version, uptime_secs, storage_backend
- ServerState: add node_id, start_time, storage_backend fields
- Health handler returns full node identity for multi-node monitoring

crates/quicproquo-server/src/domain/mod.rs

@@ -17,5 +17,6 @@ pub mod groups;
 pub mod p2p;
 pub mod account;
 pub mod moderation;
+pub mod notification;
 pub mod rate_limit;
 pub mod recovery;