Bump mongoose from 8.4.1 to 8.9.5 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 8.4.1 to 8.9.5.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@8.4.1...8.9.5)

---
updated-dependencies:
- dependency-name: mongoose
  dependency-type: direct:production
...

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Author: dependabot[bot]