chore(deps): bump google-auth-oauthlib from 1.2.0 to 1.3.1 in /backend

Bumps [google-auth-oauthlib](https://github.com/googleapis/google-cloud-python) from 1.2.0 to 1.3.1.
- [Release notes](https://github.com/googleapis/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/main/packages/gcp-sphinx-docfx-yaml/CHANGELOG.md)
- [Commits](https://github.com/googleapis/google-cloud-python/compare/kms-1.2.0...google-auth-oauthlib-v1.3.1)

---
updated-dependencies:
- dependency-name: google-auth-oauthlib
  dependency-version: 1.3.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -30,7 +30,7 @@ email-validator==2.1.0.post1
 # Gmail API (for direct email injection)
 google-api-python-client==2.193.0
 google-auth==2.49.1
-google-auth-oauthlib==1.2.0
+google-auth-oauthlib==1.3.1
 google-auth-httplib2==0.3.0
 
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