fix(co-gahusb): update_task 존재하지 않는 task_id not_found 가드

co-gahusb/app/store.py

@@ -106,6 +106,8 @@ async def update_task(r, task_id, status, role, note=None):
     if status not in VALID_STATUS:
         raise ValueError(f"invalid status: {status}")
     key = TASK_PREFIX + str(task_id)
+    if not await r.exists(key):
+        return {"ok": False, "error": "not_found"}
     mapping = {"status": status}
     if note is not None:
         mapping["note"] = note