Bump express from 4.22.2 to 5.2.1 in /backend

Bumps [express](https://github.com/expressjs/express) from 4.22.2 to 5.2.1.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@v4.22.2...v5.2.1)

---
updated-dependencies:
- dependency-name: express
  dependency-version: 5.2.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Author: dependabot[bot]