Backend: Add transaction outbox pattern for reliable event delivery (webhooks/audits)

[greatest0fallt1me]

Description

Implement the transactional outbox pattern so DB commits and downstream dispatch (webhooks, internal events) remain consistent. This prevents losing events when the process crashes after a DB write.

Requirements and context

• Must be secure, tested, and documented.
• Must be idempotent and retry-safe.

Suggested execution

git checkout -b feature/transactional-outbox

Implement changes

• Add outbox table and worker.
• Publish outbox entries within DB transaction.
• Add tests for crash/retry behavior and idempotency.

Test and commit

npm test

Example commit message

feat(reliability): transactional outbox for durable event delivery

Guidelines

• Timeframe: 96 hours.