Enforce max websocket listeners

sync-server/src/server/endpoints/websocket.rs

@@ -32,7 +32,16 @@ pub async fn websocket_handler(
 ) -> Result<Response, SyncServerError> {
     debug!("Upgrading WebSocket connection for vault `{vault_id}`");
 
+    let Some(permit) = state.broadcasts.try_admit(&vault_id) else {
+        return Ok((
+            StatusCode::TOO_MANY_REQUESTS,
+            "Vault connection limit reached",
+        )
+            .into_response());
+    };
+
     Ok(ws.on_upgrade(move |socket| async move {
+        let _permit = permit;
         if let Err(error) = websocket(state, socket, vault_id).await {
             debug!("WebSocket disconnected: {error}");
         }