fix(stream): bracket IPv6 forward host for valid nginx upstream

A Stream Host with an IPv6 address as the Forward Host was accepted and
saved but never activated. The generated stream config rendered
`proxy_pass {{ forwarding_host }}:{{ forwarding_port }}` as e.g.
`fe80::528:3c87:e7bb:ab08:25`, which nginx rejects with
"invalid port in upstream" because an IPv6 literal must be wrapped in
square brackets before the port is appended (`[fe80::...]:25`).

Normalize the stream forward host in generateConfig (alongside the existing
per-host-type data massaging) using net.isIPv6(), so IPv6 hosts render as
`[address]:port` while IPv4 addresses and hostnames are emitted unchanged.
The mutation is applied to the deep-copied render object, so persisted and
audit data are unaffected.

Fixes #5740

backend/internal/nginx.js

@@ -1,4 +1,5 @@
 import fs from "node:fs";
+import net from "node:net";
 import { dirname } from "node:path";
 import { fileURLToPath } from "node:url";
 import _ from "lodash";
@@ -221,6 +222,14 @@ const internalNginx = {
 				host.forward_scheme = "$scheme";
 			}
 
+			// A stream forwarding to an IPv6 literal must have the address wrapped in
+			// square brackets before nginx appends ":<port>". Without the brackets nginx
+			// reads the trailing ":<port>" as part of the address and rejects the upstream
+			// ("invalid port in upstream"), so the stream saves but never activates (#5740).
+			if (nice_host_type === "stream" && net.isIPv6(host.forwarding_host)) {
+				host.forwarding_host = `[${host.forwarding_host}]`;
+			}
+
 			if (host.locations) {
 				//logger.info ('host.locations = ' + JSON.stringify(host.locations, null, 2));
 				origLocations = [].concat(host.locations);