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

proto/qpq/v1/p2p.proto

@@ -23,4 +23,12 @@ message HealthRequest {}
 
 message HealthResponse {
   string status = 1;
+  // Unique node identifier for multi-node deployments.
+  string node_id = 2;
+  // Server version string.
+  string version = 3;
+  // Uptime in seconds since process start.
+  uint64 uptime_secs = 4;
+  // Storage backend type (e.g. "sql", "file", "postgres").
+  string storage_backend = 5;
 }