- Use (sourceTopic, eventId) as composite unique key in processed_events
- Add sourceTopic to ServiceEvent for globally unique idempotency key
- Wrap all handlers with withIdempotency() for duplicate event detection
- Fix ID collision issue between different service outbox tables
This implements the industry-standard CDC exactly-once semantics pattern.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>