chore(deps): bump fastapi from 0.135.2 to 0.135.3 in /backend

Bumps [fastapi](https://github.com/fastapi/fastapi) from 0.135.2 to 0.135.3.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](https://github.com/fastapi/fastapi/compare/0.135.2...0.135.3)

---
updated-dependencies:
- dependency-name: fastapi
  dependency-version: 0.135.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/requirements.txt

@@ -1,5 +1,5 @@
 # Core Framework
-fastapi==0.135.2  # Updated: Fixed starlette DoS vulnerabilities (was 0.109.1); pulls in starlette>=1.0.0
+fastapi==0.135.3  # Updated: Fixed starlette DoS vulnerabilities (was 0.109.1); pulls in starlette>=1.0.0
 uvicorn[standard]==0.43.0
 pydantic==2.12.5
 pydantic-settings==2.13.1