feat: implement cross-node event broadcasting with NATSEventBus (#151)

This PR implements cross-node event broadcasting for aether.

Changes:
- UpdateVersionCache method in JetStreamEventStore
- SubscribeToEventStored helper in NATSEventBus
- Integration tests for cross-node scenarios
- Example code demonstrating NATSEventBus + JetStreamEventStore

Tests: All integration tests passing.
Co-authored-by: Claude Code <noreply@anthropic.com>
Co-authored-by: Hugo Nijhuis <hugo.nijhuis@flowmade.one>
Reviewed-on: #151

examples/cross_node_broadcasting.go

@@ -0,0 +1,168 @@
+// Package main demonstrates cross-node event broadcasting using NATSEventBus
+// and JetStreamEventStore for cluster synchronization.
+//
+// This example shows:
+// 1. Setting up NATSEventBus with JetStreamEventStore
+// 2. Broadcasting events across NATS for cross-node distribution
+// 3. Subscribing to EventStored events for version cache synchronization
+// 4. Properly handling EventStored events from other cluster nodes
+//
+// Prerequisites:
+//   - NATS server running with JetStream enabled (nats-server -js)
+//   - Events stream created in JetStream
+package main
+
+import (
+	"context"
+	"log"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"git.flowmade.one/flowmade-one/aether"
+	"git.flowmade.one/flowmade-one/aether/store"
+	"github.com/google/uuid"
+	"github.com/nats-io/nats.go"
+)
+
+func main() {
+	natsURL := getEnv("NATS_URL", "nats://localhost:4222")
+
+	nc, err := nats.Connect(natsURL)
+	if err != nil {
+		log.Fatal("Failed to connect to NATS:", err)
+	}
+	defer nc.Close()
+
+	ctx := context.Background()
+
+	store1, err := store.NewJetStreamEventStore(nc, "events")
+	if err != nil {
+		log.Fatal("Failed to create event store:", err)
+	}
+
+	eventBus1 := aether.NewNATSEventBusWithBroadcaster(nc, store1, "")
+	defer eventBus1.Stop()
+
+	store2, err := store.NewJetStreamEventStore(nc, "events")
+	if err != nil {
+		log.Fatal("Failed to create event store:", err)
+	}
+
+	eventBus2 := aether.NewNATSEventBusWithBroadcaster(nc, store2, "")
+	defer eventBus2.Stop()
+
+	eventStoredCh1 := eventBus1.SubscribeToEventStored("*")
+	eventStoredCh2 := eventBus2.SubscribeToEventStored("*")
+
+	done := make(chan struct{})
+
+	go processEvents(ctx, eventStoredCh1, store1, done)
+	go processEvents(ctx, eventStoredCh2, store2, done)
+
+	go func() {
+		time.Sleep(2 * time.Second)
+
+		actorID := "demo-actor"
+
+		event1 := &aether.Event{
+			ID:        uuid.New().String(),
+			EventType: "OrderPlaced",
+			ActorID:   actorID,
+			Version:   1,
+			Data: map[string]interface{}{
+				"total":  99.99,
+				"status": "pending",
+			},
+			Timestamp: time.Now(),
+		}
+
+		log.Printf("Node 1 publishing event: %s", event1.EventType)
+		eventBus1.Publish("", event1)
+
+		time.Sleep(500 * time.Millisecond)
+
+		event2 := &aether.Event{
+			ID:        uuid.New().String(),
+			EventType: "OrderPaid",
+			ActorID:   actorID,
+			Version:   2,
+			Data: map[string]interface{}{
+				"total":   99.99,
+				"status":  "paid",
+				"method":  "credit_card",
+			},
+			Timestamp: time.Now(),
+		}
+
+		log.Printf("Node 2 publishing event: %s", event2.EventType)
+		eventBus2.Publish("", event2)
+
+		time.Sleep(2 * time.Second)
+
+		close(done)
+
+		log.Println("Cross-node broadcasting demo complete")
+	}()
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
+
+	select {
+	case <-sigCh:
+		log.Println("Shutting down...")
+	case <-done:
+	}
+}
+
+func processEvents(ctx context.Context, eventStoredCh <-chan *aether.Event, eventStore *store.JetStreamEventStore, done chan struct{}) {
+	for {
+		select {
+		case <-done:
+			return
+		case <-ctx.Done():
+			return
+		case event, ok := <-eventStoredCh:
+			if !ok {
+				return
+			}
+
+			if event == nil {
+				continue
+			}
+
+			if event.EventType != aether.EventTypeEventStored {
+				continue
+			}
+
+			actorID, ok := event.Data["actorId"].(string)
+			if !ok {
+				log.Printf("Warning: EventStored missing actorId")
+				continue
+			}
+
+			version, ok := event.Data["version"].(int64)
+			if !ok {
+				log.Printf("Warning: EventStored missing version")
+				continue
+			}
+
+			eventID, _ := event.Data["eventId"].(string)
+
+			log.Printf("Received EventStored: actor=%s, version=%d, eventId=%s", actorID, version, eventID)
+
+			eventStore.UpdateVersionCache(actorID, version)
+
+			currentVersion, _ := eventStore.GetLatestVersion(actorID)
+			log.Printf("Updated cache: %s now has version %d (cached: %d)", actorID, version, currentVersion)
+		}
+	}
+}
+
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}