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
Christian Nennemann
885cce0d7d