feat: ajout proxy WebSocket pour LiveKit en mode HTTPS

Permet au client PWA charge en HTTPS de se connecter a LiveKit via proxy WSS.

Modifications:
- server/index.js: ajout proxy http-proxy-middleware
- Route /livekit proxie vers http://localhost:7880
- Upgrade WebSocket active pour le proxy
- server/package.json: ajout dependance http-proxy-middleware

Fonctionnement:
- Client HTTPS se connecte a wss://localhost:3000/livekit
- Serveur Express proxie vers ws://localhost:7880
- Resout probleme mixed content (HTTPS ne peut pas WS)

server/package.json

@@ -22,6 +22,7 @@
     "@livekit/rtc-node": "^0.13.28",
     "dotenv": "^17.4.2",
     "express": "^4.19.2",
+    "http-proxy-middleware": "^4.1.1",
     "livekit-server-sdk": "^2.6.0",
     "opusscript": "^0.1.1",
     "qrcode-terminal": "^0.12.0",