implement transactional outbox pattern for reliable event publishing

[kuse-design]

Summary

This PR implements the Transactional Outbox Pattern to ensure reliable event publishing. Instead of publishing webhook and Redis Pub/Sub events directly within database transactions, events are first persisted to an outbox table as part of the same transaction. A relay worker then polls the outbox and publishes pending events asynchronously.

Changes

• Added an outbox table for persisting unpublished events
• Stored domain events in the outbox within the same database transaction as the corresponding state change
• Implemented a relay worker to poll the outbox every second
• Published pending events to the configured message broker
• Marked successfully published events as processed and removed them from the outbox
• Added safeguards to support idempotent event processing and prevent duplicate side effects
• Followed existing project patterns and transaction boundaries

Testing

• Verified events are written to the outbox as part of the originating database transaction
• Verified the relay worker publishes pending events successfully
• Verified published events are marked as processed and removed from the outbox
• Verified duplicate event delivery does not produce duplicate side effects
• Confirmed existing event publishing flows continue to function without regressions

Related Issue

Closes #121