test(webhooks): add integration test for webhook retry on 500 response (#578)

[Hollujay]

Closes #578

Summary

Adds an integration test to verify that the webhook delivery system correctly retries after receiving a 500 Internal Server Error from the callback URL, rather than immediately marking the webhook as failing.

Test Approach

1. Mock server: A local HTTP server is started that returns 500 on the first request and 200 on all subsequent requests. Request timestamps are recorded.
2. Webhook registration: A webhook is created via Prisma pointing to the mock server URL.
3. Event dispatch: dispatchWebhookEvent is called with a buy trade event, triggering delivery to the mock server.
4. Retry verification:
   • The first attempt receives a 500 → the service applies exponential backoff (Math.pow(2, 1) * 1000 = 2000ms)
   • The second attempt receives a 200 → the event is marked DELIVERED
5. Assertions:
   • Callback was called exactly twice (first fail, then success)
   • Time between calls ≥ 2000ms (correct backoff delay)
   • Webhook event status is DELIVERED with retryCount: 2
   • Webhook is not marked as isFailing: true (only flagged after all retries are exhausted)

Acceptance Criteria Met

• Callback called twice: once failing, once succeeding
• Webhook not marked as failed after successful retry
• Retry fires after the configured backoff delay

Files Changed

• src/modules/webhooks/webhook.integration.test.ts — Added the new test block with mock HTTP server and assertions