chore(deps): bump axum from 0.8.4 to 0.8.9 in /backend

Bumps [axum](https://github.com/tokio-rs/axum) from 0.8.4 to 0.8.9.
- [Release notes](https://github.com/tokio-rs/axum/releases)
- [Changelog](https://github.com/tokio-rs/axum/blob/main/CHANGELOG.md)
- [Commits](tokio-rs/axum@axum-v0.8.4...axum-v0.8.9)

---
updated-dependencies:
- dependency-name: axum
  dependency-version: 0.8.9
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]