Implement headless ComfyUI installation and improve backend status handling

- Added a new function to ensure headless installation of ComfyUI when backends are empty, enhancing the setup process for SwarmUI.
- Updated documentation to clarify the installation flow and backend readiness checks, ensuring users understand the requirements for a successful setup.
- Enhanced backend status checks to differentiate between 'empty' and 'idle' states, improving error handling and user feedback during provisioning.
- Adjusted logging messages to provide clearer insights into the installation and backend status processes.

tests/test_session.py

@@ -47,6 +47,10 @@ def _mock_bind(monkeypatch):
         lambda cfg, host, log, update=True, light=False: None,
     )
     monkeypatch.setattr("gpu_rent.session.provision_vm", lambda cfg, host, log, **kw: None)
+    monkeypatch.setattr(
+        "gpu_rent.session.ensure_swarm_comfy_installed",
+        lambda cfg, host, log: None,
+    )
     monkeypatch.setattr("gpu_rent.session.wait_backend_idle", lambda cfg, host, log, **kw: None)
     monkeypatch.setattr(
         "gpu_rent.session.verify_stack_on_vm",