db-handler: cross-check user_uuid submission create

Only thing left to do: check authorization check for
```py
@router.post("/submission")
async def create_submission(
submission: SubmissionCreate, session: SessionDep, authorization: str = Header(...)
) -> SubmissionCreateResponse:
```
endpoint: does `user_uuid` from `authorization` match `user_uuid` from `SubmissionCreate`?

However, this is not necessary for this merge, as we need to connect frontend to this as soon as possible!

_Originally posted by @OlivierSwaak in https://github.com/JoelvanIngen/Competitive-Green-Coding/issues/365#issuecomment-3006455744_