Upgrade y-websocket sidecar from v1 to @y/websocket-server (v2)

Replace the inline y-websocket@1 server hack with the official
@y/websocket-server package for proper v2 protocol support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -56,18 +56,8 @@ services:
       - sh
       - -c
       - |
-        npm init -y > /dev/null 2>&1
-        npm install y-websocket@1 ws > /dev/null 2>&1
-        node -e "
-        const ws = require('ws');
-        const Y = require('yjs');
-        const {setupWSConnection, docs} = require('y-websocket/bin/utils');
-        const host = process.env.HOST || '0.0.0.0';
-        const port = parseInt(process.env.PORT || '1234');
-        const wss = new ws.Server({ host, port });
-        wss.on('connection', setupWSConnection);
-        console.log('y-websocket server running on ' + host + ':' + port);
-        "
+        npm install @y/websocket-server > /dev/null 2>&1
+        npx y-websocket
     environment:
       - HOST=0.0.0.0
       - PORT=1234