`requirements.txt` Pulls Latest SDK Causing Version Conflicts with Label Studio

I ran into a version conflict when setting up the ML backend.

**Problem**:
The `requirements.txt` currently has:

label-studio-sdk @ git+https://github.com/HumanSignal/label-studio-sdk.git

This pulls the latest SDK from Git (currently 2.0.19).

`label-studio 1.23.0` expects `label-studio-sdk==2.0.18`.

This causes pip dependency warnings and may confuse developers trying to run the backend locally.

**Optional note for developers**:

Developers running examples outside Docker may encounter import or CLI issues if SDK versions are mismatched.

Suggested fix:

Pin `label-studio-sdk` in `requirements.txt` to a compatible version (e.g., 2.0.18) or document the version compatibility clearly.

I’d be happy to contribute a PR updating `requirements.txt` and improving the notes for developers if that’s helpful.