Implement Invoice Status Webhook Delivery

Description

Send signed webhook events for invoice lifecycle changes with retry, deduplication, and failure tracking.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested execution

• Fork the repo and create a branch
• git checkout -b feature/invoice-status-webhooks
• Implement changes
• Write backend code: webhook producer and signer
• Write comprehensive tests: signature, retry, and dedup logic
• Add documentation: README.md
• Include JSDoc-style comments
• Validate security assumptions

Test and commit

• Run tests
• Cover edge cases
• Include test output and security notes

Example commit message

feat(api): add signed webhook notifications for invoice status

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours