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.

src/gpu_rent/session.py

@@ -21,7 +21,12 @@ from gpu_rent.cloud import (
     wait_volume,
 )
 from gpu_rent.bootstrap import run_bootstrap
-from gpu_rent.provision import provision_vm, seed_swarmui_api_keys, tune_swarm_perf
+from gpu_rent.provision import (
+    ensure_swarm_comfy_installed,
+    provision_vm,
+    seed_swarmui_api_keys,
+    tune_swarm_perf,
+)
 from gpu_rent.ready import verify_gpu_env, verify_stack_on_vm, wait_backend_idle
 from gpu_rent.snapshot import ensure_boot_snapshot
 from gpu_rent.notify import notify_ready
@@ -152,10 +157,17 @@ def _bind_access(
     )
     clock.mark("provision", log)
     if swarm:
+        try:
+            ensure_swarm_comfy_installed(cfg, ip, log)
+        except Exception as exc:
+            log(f"SwarmUI Comfy install: {exc}")
+            raise
+        clock.mark("comfy-install", log)
         try:
             wait_backend_idle(cfg, ip, log)
         except CloudError as exc:
             log(f"ready: {exc}")
+            raise
         clock.mark("Idle", log)
         try:
             if tune_swarm_perf(cfg, ip, log):