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/v2_handlers/mod.rs

@@ -57,6 +57,12 @@ pub struct ServerState {
     pub banned_users: Arc<DashMap<Vec<u8>, BanRecord>>,
     /// Moderation reports (append-only).
     pub moderation_reports: Arc<std::sync::Mutex<Vec<ModerationReport>>>,
+    /// Unique node identifier for multi-node health reporting.
+    pub node_id: String,
+    /// Process start time for uptime calculation.
+    pub start_time: std::time::Instant,
+    /// Storage backend name (e.g. "sql", "file").
+    pub storage_backend: String,
 }
 
 /// A ban record for a user.