Update fastapi requirement from ~=0.135.1 to ~=0.135.3 in /backend

Updates the requirements on [fastapi](https://github.com/fastapi/fastapi) to permit the latest version.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](fastapi/fastapi@0.135.1...0.135.3)

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

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

Author: dependabot[bot]

backend/requirements.txt

@@ -1,3 +1,3 @@
-fastapi~=0.135.1
+fastapi~=0.135.3
 pillow~=12.1.1
 uvicorn~=0.42.0