build(deps-dev): bump vitest from 2.1.9 to 4.1.8 in /backend

Bumps [vitest](https://github.com/vitest-dev/vitest/tree/HEAD/packages/vitest) from 2.1.9 to 4.1.8.
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Changelog](https://github.com/vitest-dev/vitest/blob/main/docs/releases.md)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.1.8/packages/vitest)

---
updated-dependencies:
- dependency-name: vitest
  dependency-version: 4.1.8
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

backend/package.json

@@ -56,6 +56,6 @@
     "ts-node": "^10.9.2",
     "tsx": "^4.19.2",
     "typescript": "^5.9.3",
-    "vitest": "^2.1.8"
+    "vitest": "^4.1.8"
   }
 }