Bump flask from 3.0.3 to 3.1.0 in /backend/src

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](pallets/flask@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] <[email protected]>

Author: dependabot[bot]

backend/src/requirements.txt

@@ -1,6 +1,6 @@
 blinker==1.8.2
 click==8.1.7
-Flask==3.0.3
+Flask==3.1.0
 itsdangerous==2.2.0
 Jinja2==3.1.4
 MarkupSafe==3.0.1