feat(event-sourcing): Publish EventStored after successful SaveEvent

Add EventStored internal event published to the EventBus when events are
successfully persisted. This allows observability components (metrics,
projections, audit systems) to react to persisted events without coupling
to application code.

Implementation:
- Add EventTypeEventStored constant to define the event type
- Update InMemoryEventStore with optional EventBroadcaster support
- Add NewInMemoryEventStoreWithBroadcaster constructor
- Update JetStreamEventStore with EventBroadcaster support
- Add NewJetStreamEventStoreWithBroadcaster constructor
- Implement publishEventStored() helper method
- Publish EventStored containing EventID, ActorID, Version, Timestamp
- Only publish on successful SaveEvent (not on version conflicts)
- Automatically recorded in metrics through normal Publish flow

Test coverage:
- EventStored published after successful SaveEvent
- No EventStored published on version conflict
- Multiple EventStored events published in order
- SaveEvent works correctly without broadcaster (nil-safe)

Closes #61

Co-Authored-By: Claude Code <noreply@anthropic.com>

event.go

@@ -73,6 +73,14 @@ type Event struct {
 	Timestamp time.Time              `json:"timestamp"`
 }
 
+// Common event types for Aether infrastructure
+const (
+	// EventTypeEventStored is an internal event published when an event is successfully persisted.
+	// This event allows observability components (metrics, projections, audit systems) to react
+	// to persisted events without coupling to application code.
+	EventTypeEventStored = "EventStored"
+)
+
 // Common metadata keys for distributed tracing and auditing
 const (
 	// MetadataKeyCorrelationID identifies related events across services