chore(deps): bump fastapi from 0.136.3 to 0.138.0 in /backend

Bumps [fastapi](https://github.com/fastapi/fastapi) from 0.136.3 to 0.138.0.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](https://github.com/fastapi/fastapi/compare/0.136.3...0.138.0)

---
updated-dependencies:
- dependency-name: fastapi
  dependency-version: 0.138.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/pyproject.toml

@@ -15,7 +15,7 @@ dependencies = [
     "cachetools==5.5.2",
     "cryptography>=46.0.7",
     "defusedxml>=0.7.1",
-    "fastapi==0.136.3",
+    "fastapi==0.138.0",
     "feedparser==6.0.10",
     "httpx==0.28.1",
     "playwright==1.59.0",