Bump flask from 3.1.0 to 3.1.1 in /backend

Bumps [flask](https://github.com/pallets/flask) from 3.1.0 to 3.1.1.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@3.1.0...3.1.1)

---
updated-dependencies:
- dependency-name: flask
  dependency-version: 3.1.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -1,4 +1,4 @@
-Flask==3.1.0
+Flask==3.1.1
 gunicorn==23.0.0
 flask-cors==5.0.1
 gpxpy==1.6.2