chore(deps): bump httpx from 0.26.0 to 0.28.1 in /backend

Bumps [httpx](https://github.com/encode/httpx) from 0.26.0 to 0.28.1.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](https://github.com/encode/httpx/compare/0.26.0...0.28.1)

---
updated-dependencies:
- dependency-name: httpx
  dependency-version: 0.28.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -16,7 +16,7 @@ python-jose[cryptography]==3.5.0  # Updated: Fixed algorithm confusion with Open
 bcrypt==4.3.0
 python-multipart==0.0.22  # Updated: Fixed multiple vulnerabilities (was 0.0.6)
 authlib==1.6.9  # Updated: Fixed OIDC hash binding, JWE RSA1_5 padding oracle, alg:none bypass, JWK header injection (was 1.6.6)
-httpx==0.26.0
+httpx==0.28.1
 
 # Payment Processing
 stripe==14.4.1