Bump starlette from 0.38.4 to 0.40.0 in /backend in the pip group

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


Updates `starlette` from 0.38.4 to 0.40.0
- [Release notes](https://github.com/encode/starlette/releases)
- [Changelog](https://github.com/encode/starlette/blob/master/docs/release-notes.md)
- [Commits](encode/starlette@0.38.4...0.40.0)

---
updated-dependencies:
- dependency-name: starlette
  dependency-type: indirect
  dependency-group: pip
...

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