[Backend] Add a response header identifying the API version

[grantfox-oss]

Files affected (3): Backend/src/app.ts, Backend/src/routes/health.routes.ts, Backend/tests/unit/health.test.ts
Description: Routes are versioned under /api/v1, but responses do not identify the API version for clients or debugging.
Acceptance criteria:

Return an X-API-Version: v1 header.

Test the header on health or API responses.

[yusufmuhammadayodeji-netizen]

Hello, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[nurudeenheroic]

I've gone through the issue and am ready to get started.

[glorious21-coder]

I understand the expected outcome and would love to work on this. I'll provide a solution that satisfies the acceptance criteria.

[grantfox-oss]

🦊 GrantFox — @yusufmuhammadayodeji-netizen has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #35)
2. Your PR will be reviewed by the Vaulty-X maintainers

Good luck! Track your progress on GrantFox.