perf: parallelize the TURN OFF button via shared _shutdown_devices

shutdown_all() was the only remaining sequential shutdown path (TV, then
Gabi, then Gaja — 10+s with SSH timeouts). It now uses the same parallel
_shutdown_devices() as the budget tick, which gained a {dev_id: result}
return value so the button response keeps per-device details and
registry ordering. Per-device failures are isolated instead of being
silently swallowed.

devices.py

@@ -278,12 +278,25 @@ def curfew_status() -> dict:
     }
 
 
-def _shutdown_devices(device_ids: list[str]):
+def _shutdown_devices(device_ids: list[str]) -> dict[str, dict | None]:
-    """Shut down a list of devices in parallel."""
+    """Shut down a list of devices in parallel.
+
+    Returns {dev_id: turnoff result or None on failure}.
+    """
+    results: dict[str, dict | None] = {}
     if not device_ids:
-        return
+        return results
     with ThreadPoolExecutor(max_workers=len(device_ids)) as executor:
-        list(executor.submit(DEVICES[dev_id]["turnoff"]) for dev_id in device_ids)
+        futures = {
+            executor.submit(DEVICES[dev_id]["turnoff"]): dev_id
+            for dev_id in device_ids
+        }
+        for future, dev_id in futures.items():
+            try:
+                results[dev_id] = future.result()
+            except Exception:
+                results[dev_id] = None
+    return results
 
 
 # ── Budget timer (runs every 10s in background) ──────────────────────
@@ -415,10 +428,14 @@ def stop_timer():
 # ── Orchestrator ──────────────────────────────────────────────────────
 
 def shutdown_all() -> list[dict]:
-    """Indiscriminately turn off all devices. Returns ordered list of action dicts."""
+    """Indiscriminately turn off all devices in parallel.
+
+    Returns ordered list of action dicts (registry order).
+    """
+    results = _shutdown_devices(list(DEVICES))
     actions = []
     for dev_id, dev_info in DEVICES.items():
-        result = dev_info["turnoff"]()
+        result = results.get(dev_id)
         if result:
             budget_minutes = _budget_to_minutes(_get_budget(dev_id))
             actions.append({