Bump fastapi from 0.109.2 to 0.115.8 in /backend

Bumps [fastapi](https://github.com/fastapi/fastapi) from 0.109.2 to 0.115.8.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](fastapi/fastapi@0.109.2...0.115.8)

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

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

Author: dependabot[bot]

backend/requirements-docker.txt

@@ -1,6 +1,6 @@
 ariadne==0.22
 asyncpg==0.29.0
-fastapi==0.109.2
+fastapi==0.115.8
 httpx==0.26.0
 pydantic==2.6.1
 python-multipart==0.0.9

backend/src/requirements_docker.txt

@@ -1,5 +1,5 @@
 ariadne==0.21
-fastapi==0.109.2
+fastapi==0.115.8
 httpx==0.26.0
 psycopg2-binary==2.9.9
 pydantic==2.5.3