Add a stream relay (TURN) to the proxy for browsers that cannot connect directly

Vanadium forbids direct UDP for WebRTC, and mobile and company networks often
block direct connections; the only route then is a relay reached over TCP.

- server/turn.ts: STUN and TURN on one port, over UDP and TCP, with
  short-lived credentials from /api/turn, quotas and a peer filter
- The browser build fetches credentials and offers the relay automatically
- Stats for nerds says when a stream is relayed and how the relay is reached
- Tests: a TURN client over UDP and TCP, and a browser limited to the relay
  over TCP in the web E2E

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/main.ts

@@ -12,7 +12,9 @@ try {
   console.log(`mumh5 proxy listening on http://${config.bind}:${port}`);
   console.log(config.allowAny ? `Allowed servers: any${config.allowPrivate ? ', private addresses included' : ' public address'}` : `Allowed servers: ${config.servers.map(s => `${s.host}:${s.port}`).join(', ')}`);
   if (config.sendProxy) console.log('Announcing client addresses with the PROXY protocol; the allowed servers must expect it');
-  console.log(stunPort ? `STUN for screen sharing on UDP port ${stunPort} (must be reachable from the internet)` : 'STUN is off; screen sharing between browser users will only work on the same network');
+  console.log(!stunPort ? 'STUN is off; screen sharing between browser users will only work on the same network'
+    : config.turn ? `Screen sharing: STUN and relay on port ${stunPort} (UDP and TCP), relayed streams on UDP ${config.turnMinPort}-${config.turnMaxPort}; all must be reachable from the internet`
+    : `Screen sharing: STUN on UDP port ${stunPort} (must be reachable from the internet), no relay`);
   console.log(config.staticDir ? `Serving the web app from ${config.staticDir}` : 'No web build found (npm run build:web); serving the API only');
 } catch (e) {
   console.error((e as Error).message);