test(webhooks): cover bounded retry and safe dead letters

[flowpatch-reliability]

Closes #75. Links back to the integration-platform parent #34.

This adds the requested deterministic failure-path coverage and tightens the durable record contract:

• timeout and 5xx failures retry with bounded backoff
• non-retryable 4xx failures dead-letter immediately
• success after retry reuses the same X-Posnic-Delivery id (documenting at-least-once duplicate expectations)
• permanent 5xx failure stops at MAX_ATTEMPTS
• durable failures store only stable error codes, not raw exception text
• dead-letter rows remove shop/time payload fields and record deadLetteredAt

Validation performed:

• node --check api/src/realtime/webhooks.js
• node --check api/tests/unit/realtime/webhooks.test.js
• standalone Node failure-path harness covering 400, 500, retry-success, stable delivery IDs, and permanent failure

The container did not include npm, so I could not run the repository Jest command locally.