Bump python-multipart from 0.0.20 to 0.0.21 in /back-end

Bumps [python-multipart](https://github.com/Kludex/python-multipart) from 0.0.20 to 0.0.21.
- [Release notes](https://github.com/Kludex/python-multipart/releases)
- [Changelog](https://github.com/Kludex/python-multipart/blob/master/CHANGELOG.md)
- [Commits](Kludex/python-multipart@0.0.20...0.0.21)

---
updated-dependencies:
- dependency-name: python-multipart
  dependency-version: 0.0.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

back-end/requirements.txt

@@ -9,7 +9,7 @@ idna==3.11
 immutables==0.21
 importlib-metadata==8.7.0
 pydantic==2.12.5
-python-multipart==0.0.20
+python-multipart==0.0.21
 six==1.17.0
 sniffio==1.3.1
 starlette==0.48.0