Bump marshmallow from 3.26.2 to 4.3.1 in /backend

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 3.26.2 to 4.3.1.
- [Changelog](https://github.com/marshmallow-code/marshmallow/blob/dev/CHANGELOG.rst)
- [Commits](https://github.com/marshmallow-code/marshmallow/compare/3.26.2...4.3.1)

---
updated-dependencies:
- dependency-name: marshmallow
  dependency-version: 4.3.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/requirements.txt

@@ -83,7 +83,7 @@ tzlocal>=5.0
 
 # OpenAPI documentation
 apispec==6.4.0
-marshmallow==3.26.2
+marshmallow==4.3.1
 
 # AI assistant (core primitive — powered by Prompture)
 # Multi-provider LLM orchestration: Conversation, ToolRegistry, ask_live streaming,