Add webhook notification delivery system for on-chain settlement status updates

[Cjay-Cyber-2]

Description

When an expense settlement transaction transitions from pending to confirmed on-chain, external applications or notification handlers need to be informed. We need to implement a webhook notification service that dispatches signed HTTP payloads upon settlement state changes.

Context & Requirements

Background settlement processing occurs asynchronously in src/worker/. Once a Stellar payment XDR containing the MP:<code> memo is validated and confirmed on the Stellar ledger, subscribers or configured group webhook endpoints should receive a notification.

Requirements:

• Implement src/services/webhook.ts to construct and send HTTP POST webhook payloads for events (e.g., settlement.confirmed, settlement.failed).
• Sign every outgoing webhook request payload using HMAC-SHA256 signature passed in the X-Mergepay-Signature header.
• Dispatch webhooks asynchronously from the background settlement worker (src/worker/).
• Ensure delivery timeouts (5 seconds) and handle HTTP errors gracefully without blocking the background worker execution.

Acceptance Criteria

• Confirmed and failed settlements trigger webhook dispatch events.
• Webhook requests include X-Mergepay-Signature header for payload authenticity verification.
• Webhook delivery failures log warnings without crashing worker execution loops.
• Unit tests verify payload creation, HMAC signature calculation, and failed request handling.

Implementation Guidance

• Create src/services/webhook.ts using node-fetch (or global fetch) to dispatch payloads.
• Integrate webhook calls into background worker processing logic in src/worker/ after database status updates.
• Store signing secret in environment config.

Testing & Validation

• Add unit tests in tests/services/webhook.test.ts using HTTP mocking to verify request headers and payload content.
• Verify HMAC signature calculation against test keys.
• Run existing test suites via npm test.

Submission Guidelines

• Include "Closes #" in the PR description.
• Wait for assignment by maintainers prior to working on the issue.
• Follow standard codebase formatting and TypeScript rules.

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Wave complexity: Medium

[Kureszn]

@Kureszn has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I've reviewed the issue and believe I can contribute effectively. My experience spans software development, blockchain technologies, cybersecurity and open source collaboration. I take a structured approach to contributions carefully understanding the existing codebase, identifying the root cause and delivering a thoroughly tested implementation.

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

[Sureplus30]

@Sureplus30 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

i would like to work on this kindly assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Sureplus30 to this issue.