fix: use wss:// LiveKit URL in federated call relay, not https://

sendFederatedCallStart was sending `https://${domain}/livekit` as the
LiveKit URL. The LiveKit SDK requires `wss://` for WebSocket connections.
The caller (local) worked because it gets the URL from config.livekit.url
(wss://). The federated acceptor failed because it used the relay URL
(https://) — the SDK can't connect over HTTPS.

Now uses config.livekit.url directly, falling back to wss:// if unset.

packages/server/src/ws/events.ts

@@ -1749,7 +1749,10 @@ async function sendFederatedCallStart(
     return;
   }
 
-  const livekitUrl = `https://${config.domain}/livekit`;
+  // Use the configured LiveKit URL (wss://domain/livekit from .env).
+  // Previously this was `https://${config.domain}/livekit` which fails because
+  // the LiveKit SDK requires a wss:// WebSocket URL, not https://.
+  const livekitUrl = config.livekit.url ?? `wss://${config.domain}/livekit`;
 
   // Build participants array (all member identities for Path B)
   const participants = members.map(m => ({