Bump express from 4.21.0 to 5.1.0 in /backend

Bumps [express](https://github.com/expressjs/express) from 4.21.0 to 5.1.0.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](expressjs/express@4.21.0...v5.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]