Replace free TURN with self-hosted coturn; fix Safari connectivity; release v5.4.10

- config/ice-servers.prod.js: swap ExpressTURN for self-hosted coturn at
  turn.securebit.chat (TURN udp/tcp on 3478, TURNS/TLS on 443). Long-lived
  REST-API credential (expiry capped at int32 max for coturn compatibility).
- Add raw-IP STUN/TURN fallback (144.172.96.126): Safari's WebRTC layer fails
  to resolve STUN/TURN hostnames on some networks and gathers no srflx/relay
  candidates; reaching the server by IP fixes cross-browser (Safari<->Chrome)
  connections. Harmless to other browsers.
- deploy/nginx.conf: never long-cache /config/ice-servers.js so clients don't
  lock onto a stale server list.
- Bump version to 5.4.10 (header + init banner).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.4.5",
+  "version": "5.4.10",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {