build(deps): bump python-multipart from 0.0.18 to 0.0.20 in /backend

Bumps [python-multipart](https://github.com/Kludex/python-multipart) from 0.0.18 to 0.0.20.
- [Release notes](https://github.com/Kludex/python-multipart/releases)
- [Changelog](https://github.com/Kludex/python-multipart/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Kludex/python-multipart/compare/0.0.18...0.0.20)

---
updated-dependencies:
- dependency-name: python-multipart
  dependency-version: 0.0.20
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/requirements.txt

@@ -1,7 +1,7 @@
 fastapi==0.115.7
 uvicorn[standard]==0.34.0
 pydantic==2.10.6
-python-multipart==0.0.18
+python-multipart==0.0.20
 
 python-socketio==5.11.3
 python-jose==3.4.0