build(deps-dev): bump black from 25.12.0 to 26.3.0 in /backend

Bumps [black](https://github.com/psf/black) from 25.12.0 to 26.3.0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@25.12.0...26.3.0)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.3.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]