add /ping endpoint for latency diagnosis

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Song Meo

nestor/rest_api.py

@@ -371,6 +371,12 @@ def get_devices(
     return DevicesResponse(devices=devices)
 
 
+@router.get("/cf3d/api/v1/ping")
+async def ping():
+    """Trivial async endpoint for latency diagnosis."""
+    return {"ok": True}
+
+
 @router.get(
     "/cf3d/api/v1/boots",
     response_model=BootsResponse,