[codex] Add GPU plugin backend template

[den-sq]

Summary

• add an optional CUDA-based Dockerfile.gpu to the plugin template
• add a compose.gpu.yml override that requests NVIDIA GPU devices for plugin backends
• document how GPU-backed plugins should keep CUDA dependencies in plugin images instead of making the core Ouroboros server image GPU-first

Refs #78

Validation

• git -C ouroboros diff --check

Not run

• docker compose -f plugins/plugin-template/backend/compose.yml -f plugins/plugin-template/backend/compose.gpu.yml config could not run because Docker is unavailable in this WSL distro.