build(deps): bump flask from 3.0.3 to 3.1.0 in /backend

Bumps [flask](https://github.com/pallets/flask) from 3.0.3 to 3.1.0.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](https://github.com/pallets/flask/compare/3.0.3...3.1.0)

---
updated-dependencies:
- dependency-name: flask
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -3,7 +3,7 @@ uvicorn[standard]==0.30.6
 pydantic==2.9.2
 python-multipart==0.0.18
 
-Flask==3.0.3
+Flask==3.1.0
 Flask-Cors==5.0.0
 
 python-socketio==5.11.3