chore(deps): bump authlib from 1.4.1 to 1.6.1 in /backend

Bumps [authlib](https://github.com/authlib/authlib) from 1.4.1 to 1.6.1.
- [Release notes](https://github.com/authlib/authlib/releases)
- [Changelog](https://github.com/authlib/authlib/blob/main/docs/changelog.rst)
- [Commits](https://github.com/authlib/authlib/compare/v1.4.1...v1.6.1)

---
updated-dependencies:
- dependency-name: authlib
  dependency-version: 1.6.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -97,7 +97,7 @@ onnxruntime==1.20.1
 faster-whisper==1.1.1
 
 PyJWT[crypto]==2.10.1
-authlib==1.4.1
+authlib==1.6.1
 
 black==25.1.0
 langfuse==2.44.0