chore(deps): bump fastapi from 0.121.0 to 0.135.2 in /backend

Bumps [fastapi](https://github.com/fastapi/fastapi) from 0.121.0 to 0.135.2.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](fastapi/fastapi@0.121.0...0.135.2)

---
updated-dependencies:
- dependency-name: fastapi
  dependency-version: 0.135.2
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -1,4 +1,4 @@
-fastapi==0.121.0
+fastapi==0.135.2
 uvicorn[standard]==0.34.0
 asyncpg==0.30.0
 sqlalchemy[asyncio]==2.0.36