Add Idempotency Key Support for Invoice Creation

[mikewheeleer]

Implement Idempotent Invoice Create Endpoint

Description

Support idempotency keys on invoice creation to prevent duplicate invoice entries during retries and network failures.

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-idempotency
• Implement changes
• Write backend code: idempotency middleware/store
• Write comprehensive tests: duplicate request handling
• 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 idempotency key support for invoice creation

Guidelines

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