chore(backend): bump vitest from 3.2.4 to 4.0.15 in /services/backend

Bumps [vitest](https://github.com/vitest-dev/vitest/tree/HEAD/packages/vitest) from 3.2.4 to 4.0.15.
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.0.15/packages/vitest)

---
updated-dependencies:
- dependency-name: vitest
  dependency-version: 4.0.15
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

services/backend/package.json

@@ -77,7 +77,7 @@
     "ts-node": "^10.9.2",
     "typescript": "^5.9.2",
     "typescript-eslint": "^8.48.0",
-    "vitest": "^3.2.4",
+    "vitest": "^4.0.15",
     "webpack": "^5.101.3",
     "webpack-cli": "^6.0.1",
     "webpack-node-externals": "^3.0.0"