Deploy a stack

Validate compose YAML, store it under the managed stacks directory, and bring the stack up. Requires the **operator** role; bind mounts, devices, privileged mode, host network, PID, IPC, or user namespaces, and the escalating capabilities listed under `RunSpec.capAdd` additionally require the **admin** role.

curl -X POST "https://example.com/api/stacks/compose" \  -H "Content-Type: application/json" \  -d '{    "name": "string",    "content": "string"  }'