Add Signed Callback Delivery to Backend (Webhook Client)

[Cedarich]

Complexity: Hard (200)

Description
Deliver verification results to the backend via a signed webhook with retries and dead-letter behavior.

Acceptance Criteria

• HMAC signature on outbound requests.
• Retry with exponential backoff and max attempts.
• Failed deliveries recorded for later replay.

[Olasunkanmi975]

@Olasunkanmi975 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please I would love to solve this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Olasunkanmi975 to this issue.

[drips-wave]

Congratulations, @Olasunkanmi975! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Olasunkanmi975: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @Olasunkanmi975 as part of the Stellar Wave Program's 6th Wave 🥳

Moderator's reason: [Auto-assessed] The PR comprehensively implements the required signed webhook delivery system, including HMAC-SHA256 signing, exponential backoff retries via BullMQ, and persistence of failed deliveries for later replay. The contribution is high-quality, featuring database migrations, new services, and E2E tests that directly address the issue's requirements.

😎 @Olasunkanmi975: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.

ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.