Enhance backend recovery and diagnostics in installation and tuning scripts

- Improved the `install_swarm_comfy` function to handle empty backend states more effectively, introducing recovery mechanisms and enhanced logging for better visibility.
- Updated the `tune_swarm_perf` function to always sanitize backend FDS corruption, ensuring consistent performance tuning.
- Added new tests to validate the functionality of backend recovery and FDS sanitization, ensuring robustness in backend management.

src/gpu_rent/remote/install_swarm_comfy.py

@@ -657,11 +657,21 @@ def main() -> int:
         return 0
 
     if comfy_venv_ok() and bstat == "empty":
-        if installed is not True:
-            print("venv есть, IsInstalled=false — запускаю InstallConfirmWS")
+        print(
+            "backend empty при venv/IsInstalled — recover (sanitize FDS / AddNewBackend)…",
+            flush=True,
+        )
+        bstat = recover_empty_backends()
+        if bstat not in ("empty", "unknown") and not bstat.startswith("error:"):
+            print(f"recovered empty → {bstat}")
+            return 0
+        if installed is True:
+            print(
+                "WARN: всё ещё empty после recover — пробую InstallConfirmWS",
+                flush=True,
+            )
         else:
-            run_diagnostics()
-            return 1
+            print("venv есть, IsInstalled=false — запускаю InstallConfirmWS")
 
     marker = DATA / ".gpu-rent-comfy-installing"
     try:

src/gpu_rent/remote/tune_swarm_perf.py

@@ -206,17 +206,21 @@ def main() -> int:
         except json.JSONDecodeError:
             prev = {}
     same_gpu = bool(prev.get("uuid") and prev.get("uuid") == plan["uuid"])
+    # Always repair FDS corruption even when tune is otherwise a no-op.
+    fds_fixed = sanitize_backends_fds()
     if same_gpu and prev.get("extra_args") == plan["extra_args"]:
         if prev.get("pip_ok") or not plan["use_sage"]:
             print(f"perf tune already applied for {plan['name']} ({plan['tier']})")
+            if fds_fixed:
+                print("RESTART_SWARMUI=1")
             return 0
 
     if same_gpu and plan["use_sage"] and prev.get("pip_ok") is False:
         print("perf tune: retry (previous pip_ok=false — sage/triton ещё не встали)")
 
     print(f"perf tune: {plan['name']} tier={plan['tier']} sage={plan['use_sage']}")
+    restarted_needed = bool(fds_fixed)
     pip_ok = not plan["use_sage"]
-    restarted_needed = False
     if plan["use_sage"]:
         py = find_comfy_python()
         if py: