Bump authlib from 1.6.5 to 1.6.6 in /backend

Bumps [authlib](https://github.com/authlib/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](https://github.com/authlib/authlib/compare/v1.6.5...v1.6.6)

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

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

backend/requirements.txt

@@ -14,7 +14,7 @@ asyncpg==0.29.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
 python-multipart==0.0.22  # Updated: Fixed multiple vulnerabilities (was 0.0.6)
-authlib==1.6.5  # Updated: Fixed algorithm confusion and DoS vulnerabilities (was 1.3.0)
+authlib==1.6.6  # Updated: Fixed algorithm confusion and DoS vulnerabilities (was 1.3.0)
 httpx==0.26.0
 
 # Payment Processing