build(deps): bump httpx-oauth from 0.15.1 to 0.16.0 in /backend

Bumps [httpx-oauth](https://github.com/frankie567/httpx-oauth) from 0.15.1 to 0.16.0.
- [Release notes](https://github.com/frankie567/httpx-oauth/releases)
- [Commits](frankie567/httpx-oauth@v0.15.1...v0.16.0)

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

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