Bump multer from 2.0.2 to 2.1.1 in /backend (#107)

Bumps [multer](https://github.com/expressjs/multer) from 2.0.2 to 2.1.1.
- [Release notes](https://github.com/expressjs/multer/releases)
- [Changelog](https://github.com/expressjs/multer/blob/main/CHANGELOG.md)
- [Commits](expressjs/multer@v2.0.2...v2.1.1)

---
updated-dependencies:
- dependency-name: multer
  dependency-version: 2.1.1
  dependency-type: direct:production
...

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