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

backend/src/utils/convex/client.py

@@ -7,14 +7,18 @@ 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
+        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
+        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
+        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

frontend/src/routes/+layout.svelte

@@ -2,13 +2,15 @@
 	import './layout.css';
 	import favicon from '$lib/assets/favicon.svg';
 	import { PUBLIC_CONVEX_URL } from '$env/static/public';
+	import { env } from '$env/dynamic/public';
 	import { setupConvex } from 'convex-svelte';
 	import { hasWebSocketSupport, setupPollingConvex } from '$lib/convex-polling.svelte';
 	import { setContext } from 'svelte';
 
 	let { children } = $props();
 
-	const usePolling = !hasWebSocketSupport();
+	const fallbackEnabled = env.PUBLIC_FALLBACK === '1';
+	const usePolling = fallbackEnabled && !hasWebSocketSupport();
 	setContext('convex-use-polling', usePolling);
 
 	if (usePolling) {