Enhance backend management and diagnostics in install_swarm_comfy.py

- Introduced a new `delete_backend` function to facilitate backend removal during recovery processes.
- Improved the `recover_errored_backends` function to handle backend reconfiguration and recreation more effectively, including enhanced logging for diagnostics.
- Added a new `add_comfy_selfstart` function to streamline the addition of new backends.
- Updated the `comfy_start_script` function to prefer absolute paths for better reliability.
- Enhanced diagnostics in `swarm_diag.py` to include detailed backend settings and mount status.
- Added tests to validate the new backend management features and ensure robust error handling.

src/gpu_rent/remote/swarm_diag.py

@@ -97,6 +97,14 @@ def swarm_api_bits() -> str:
                         "current_model": val.get("current_model"),
                     }
             chunks.append("ListBackends=" + json.dumps(summary, ensure_ascii=False, indent=2))
+            # Full settings for first backend (StartScript path matters)
+            for key, val in (backends or {}).items():
+                if isinstance(val, dict) and val.get("settings"):
+                    chunks.append(
+                        f"backend[{key}].settings="
+                        + json.dumps(val.get("settings"), ensure_ascii=False)
+                    )
+                    break
         except Exception as exc:
             chunks.append(f"ListBackends fail: {exc}")
     except Exception as exc:
@@ -109,10 +117,14 @@ def paths_bits() -> str:
         f"DATA={DATA} exists={DATA.is_dir()}",
         f"dlbackend={DLBACKEND} exists={DLBACKEND.is_dir()}",
         f"ComfyUI={COMFY} exists={COMFY.is_dir()}",
+        f"main.py={(COMFY / 'main.py')} exists={(COMFY / 'main.py').is_file()}",
         f"venv_python={VENV_PY} exists={VENV_PY.is_file()} exec={os.access(VENV_PY, os.X_OK) if VENV_PY.is_file() else False}",
+        f"opt_dlbackend_main={Path('/opt/swarmui/dlbackend/ComfyUI/main.py')} "
+        f"exists={Path('/opt/swarmui/dlbackend/ComfyUI/main.py').is_file()}",
         f"Settings.fds exists={SETTINGS.is_file()}",
         f"Backends.fds exists={BACKENDS_FDS.is_file()}",
     ]
+    rows.append("findmnt /opt/swarmui/dlbackend:\n" + _run(["findmnt", "/opt/swarmui/dlbackend"]))
     if SETTINGS.is_file():
         try:
             for line in SETTINGS.read_text(encoding="utf-8", errors="replace").splitlines():