fix: use shared webhook retry delay helper

[nomsoscript]

Closes #597

Changes

• Replaces the webhook retry inline delay calculation with the shared computeRetryDelay helper.
• Uses the configured webhook base retry delay and caps retry backoff at 30 seconds.
• Pins jitter in the retry unit test for deterministic timer assertions.
• Strengthens retry exhaustion assertions to verify the structured error log emits exactly once and does not include the callback URL.

Testing

• git diff --check
• pnpm exec jest src/utils/__tests__/retry-delay.utils.test.ts src/modules/webhooks/webhook.service.test.ts --runInBand attempted locally, but this machine timed out while pnpm was still materializing dependencies before Jest started.