fix(backend): add async lock to prevent Already borrowed issue

backend/src/utils/convex/client.py

@@ -7,15 +7,19 @@ from convex import ConvexClient as SyncConvexClient
 class ConvexClient:
     def __init__(self, url: str) -> None:
         self._client = SyncConvexClient(url)
+        self._lock = asyncio.Lock()
 
     async def query(self, name: str, args: dict[str, Any] | None = None) -> Any:  # noqa: ANN401
-        return await asyncio.to_thread(self._client.query, name, args or {})
+        async with self._lock:
+            return await asyncio.to_thread(self._client.query, name, args or {})
 
     async def mutation(self, name: str, args: dict[str, Any] | None = None) -> Any:  # noqa: ANN401
-        return await asyncio.to_thread(self._client.mutation, name, args or {})
+        async with self._lock:
+            return await asyncio.to_thread(self._client.mutation, name, args or {})
 
     async def action(self, name: str, args: dict[str, Any] | None = None) -> Any:  # noqa: ANN401
-        return await asyncio.to_thread(self._client.action, name, args or {})
+        async with self._lock:
+            return await asyncio.to_thread(self._client.action, name, args or {})
 
     def subscribe(self, name: str, args: dict[str, Any] | None = None) -> Any:  # noqa: ANN401
         return self._client.subscribe(name, args or {})