test(webhooks): cover hmac signing and verification

[EmmyKay0026]

Closes #216

---

Summary

Implemented full HMAC signing and verification for webhook endpoints.

Details

• Added HMAC generation logic to sign outgoing webhook payloads.
• Implemented HMAC verification middleware to validate incoming webhook requests.
• Updated related unit tests to cover both signing and verification scenarios.
• Modified documentation to include usage examples and security considerations.

Motivation

Ensures integrity and authenticity of webhook communications, protecting against tampering and replay attacks.

Testing

• New tests confirm correct HMAC generation and successful verification for valid signatures.
• Manual verification with sample payloads confirms end‑to‑end functionality.

Checklist

• Code follows project style guidelines
• Added/updated tests
• Updated documentation
• Linter and type checks pass

[drips-wave]

@EmmyKay0026 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits