⬆️ [bump][backend] Update simplejson requirement in /backend

Updates the requirements on [simplejson](https://github.com/simplejson/simplejson) to permit the latest version.
- [Release notes](https://github.com/simplejson/simplejson/releases)
- [Changelog](https://github.com/simplejson/simplejson/blob/main/CHANGES.txt)
- [Commits](simplejson/simplejson@v3.17.6...v4.1.1)

---
updated-dependencies:
- dependency-name: simplejson
  dependency-version: 4.1.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -16,7 +16,7 @@ dependencies = [
     "python-multipart>=0.0.32",
     "loguru>=0.7.3",
     "tzdata>=2026.2",
-    "simplejson (>=3.17.6,<4.0.0)",
+    "simplejson (>=3.17.6,<5.0.0)",
     "pydantic-settings (>=2.7.1,<3.0.0)",
     "pydantic-extra-types (>=2.10.2,<3.0.0)",
     "psycopg[binary]>=3.3.4",