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).

sw.js

@@ -11,7 +11,7 @@ let DYNAMIC_CACHE = 'securebit-pwa-dynamic-v4.7.56';
 // Build stamp — rewritten by scripts/post-build.js on every release so this file's
 // bytes change each deploy. That is what makes the browser detect a new Service Worker,
 // reinstall it, drop stale caches and (via controllerchange) prompt the page to update.
-const SW_BUILD_VERSION = '1784694927450';
+const SW_BUILD_VERSION = '1784757947117';
 
 // Load version from meta.json on install
 async function getAppVersion() {