Bump black from 25.1.0 to 26.3.1 in /example/backend

Bumps [black](https://github.com/psf/black) from 25.1.0 to 26.3.1.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@25.1.0...26.3.1)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.3.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]

example/backend/poetry.lock

@@ -18,7 +18,7 @@ anaconda = "^0.0.1.1"
 jedi = "^0.19.2"
 ruff = "^0.12.11"
 mypy = "^1.17.1"
-black = "^25.1.0"
+black = ">=25.1,<27.0"
 autohooks-plugin-mypy = "^23.10.0"
 autohooks-plugin-ruff = "^25.3.1"
 autohooks-plugin-black = "^23.10.0"