docs: add header comment for replace_logger script

[RohitMandal-cs]

##Summary

• Added a header comment explaining the purpose of backend/replace_logger.cjs.
• Documented that it is a one-time migration script and not part of the application runtime.

Fixes #1507

Pull Request Checklist

Please ensure your PR follows these steps, mirroring our CONTRIBUTING.md guidelines.

• I have read the CONTRIBUTING.md document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation accordingly.
• I have verified the changes locally.

[ogazboiz]

clear, truthful header comment, verified it matches what replace_logger.cjs actually does (regex logger call rewrite over a hardcoded file list, not imported at runtime). merging.

if you want to keep contributing, join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0