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).

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1784694927450",
-  "buildVersion": "1784694927450",
-  "appVersion": "5.4.5",
-  "buildTime": "2026-07-22T04:35:27.501Z",
-  "buildId": "1784694927450-0e3e3a2",
-  "gitHash": "0e3e3a2",
+  "version": "1784757947117",
+  "buildVersion": "1784757947117",
+  "appVersion": "5.4.10",
+  "buildTime": "2026-07-22T22:05:47.156Z",
+  "buildId": "1784757947117-c98a01b",
+  "gitHash": "c98a01b",
   "generated": true,
-  "generatedAt": "2026-07-22T04:35:27.502Z"
+  "generatedAt": "2026-07-22T22:05:47.157Z"
 }