Bump authlib in /backend in the pip group across 1 directory

Bumps the pip group with 1 update in the /backend directory: [authlib](https://github.com/authlib/authlib).


Updates `authlib` from 1.6.5 to 1.6.6
- [Release notes](https://github.com/authlib/authlib/releases)
- [Changelog](https://github.com/authlib/authlib/blob/main/docs/changelog.rst)
- [Commits](authlib/authlib@v1.6.5...v1.6.6)

---
updated-dependencies:
- dependency-name: authlib
  dependency-version: 1.6.6
  dependency-type: direct:production
  dependency-group: pip
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/requirements.txt

@@ -37,4 +37,4 @@ typer==0.15.2
 tzdata==2025.2
 uvicorn==0.34.0
 validators==0.34.0
-Authlib==1.6.5
+Authlib==1.6.6