[huangji6693-max]

Summary

Adds deterministic unit test coverage in backend/src/services/__tests__/webhookRetryProcessor.test.ts for webhook retry dead-letter persistence, replay, and cron telemetry in backend/src/services/webhookRetryProcessor.ts and backend/src/services/webhookService.ts:

• Max retry attempts & network error dead-lettering: Verifies that when a retry reaches MAX_RETRY_ATTEMPTS (attempt_count = 4) on either HTTP failure or network exception (ECONNRESET), next_retry_at is set to NULL so the delivery is dead-lettered and excluded from future polling.
• Dead-letter inspection: Verifies getSubscriptionDeliveries exposes dead-lettered delivery records with attemptCount = 4, lastStatusCode, lastError, and deliveredAt = undefined.
• Dead-letter replay: Verifies that when an operator re-queues a dead-lettered delivery (attempt_count < MAX_RETRY_ATTEMPTS, next_retry_at <= now), WebhookService.processRetries() delivers the payload, sets delivered_at, and clears next_retry_at = NULL.
• Retry processor lock recovery: Verifies that if refreshWebhookRetryQueueDepth throws during a scheduled tick, jobMetricsService.recordFailure is recorded and the inFlight lock is released in finally so subsequent ticks continue processing.

Closes #2077

Verification

• node --experimental-vm-modules node_modules/jest/bin/jest.js src/services/__tests__/webhookRetryProcessor.test.ts: 15 passed, 0 failed.

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[github-actions]

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• backend/src/services/__tests__/webhookRetryProcessor.test.ts — hidden-appended-config-payload (high)

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