Implement EventStored infrastructure event that notifies subscribers when an event
is successfully persisted. This enables observability and triggers downstream
workflows (caching, metrics, projections) without coupling to application events.
Changes:
- Add EventStored type to event.go containing EventID, ActorID, Version, Timestamp
- Update InMemoryEventStore with optional EventBus and metrics support via builder methods
- Update JetStreamEventStore with optional EventBus and metrics support via builder methods
- Publish EventStored to __internal__ namespace after successful SaveEvent
- EventStored not published if SaveEvent fails (e.g., version conflict)
- EventStored publishing is optional - stores work without EventBus configured
- Metrics are recorded for each EventStored publication
- Add comprehensive test suite covering all acceptance criteria
Meets acceptance criteria:
- EventStored published after SaveEvent succeeds
- EventStored contains EventID, ActorID, Version, Timestamp
- No EventStored published if SaveEvent fails
- EventBus receives EventStored in same operation
- Metrics increment for each EventStored published
Closes#61
Co-Authored-By: Claude Code <noreply@anthropic.com>
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