Update pytest-cov requirement from 4.0.0 to 6.0.0 in /backend

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v4.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:development
...

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

backend/Pipfile

@@ -39,7 +39,7 @@ pluggy = "1.5.0"
 pylint = "3.3.1"
 pytest = "8.3.4"
 pytest-asyncio = "0.21.2"
-pytest-cov = "4.0.0"
+pytest-cov = "6.0.0"
 pytest-xdist = "3.6.1"
 ruff = "0.9.1"
 vulture = "2.14"