Prometheus on a node; panel under a URL prefix; open ports on migrate import

A panel without VPN (e.g. after the move) can use a Prometheus that lives on
a node, such as the one with history left on the old server (node #0). Both
directions go through the agent's WSS: the node's Prometheus scrapes
127.0.0.1:AGENT_METRICS_PORT/metrics, the agent checks its token and asks
core (metrics → metrics_res, the same exposition as /metrics, at most once
per 5 s per connection); the monitor tab reads it back through the agent,
which forwards only GET /-/ready and /api/v1/query[_range]. The panel finds
it by itself (local first, then the lowest-id node), and the install/remove
buttons work on a panel without VPN. `monitor install` on a node without its
own panel picks AGENT_METRICS_PORT, writes it to cli.env and restarts the
agent; on the old server it keeps the collected data.

The panel works behind nginx in a subfolder: relative asset and API paths,
relative directory redirects, and the client address from X-Forwarded-For /
X-Real-IP only from trusted proxies (loopback, TRUST_PROXY).

migrate import opens the moved SUB_PORT/NODE_PORT in UFW when it is active.

internal/ui/migrate.go

@@ -280,6 +280,10 @@ func (m *Migration) importHandler(c *Ctx) bool {
 	if err := envfile.Set(m.envFile, plan.Env); err != nil {
 		return fail(err)
 	}
+	// Подписка и хаб переезжают на порты старого сервера — их надо открыть, как install.sh.
+	if opened, failed := migrate.OpenFirewall(plan.SubPort, plan.NodePort); len(opened)+len(failed) > 0 {
+		logf("migrate: UFW открыт %v, не открылись %v", opened, failed)
+	}
 	join := migrate.JoinString(host, plan.NodePort, pin, 0, secret, plan.UIPort, plan.SubPort)
 	m.state = moveImported
 	logf("migrate: панель «%s» принята с %s — перезапуск", b.Str("server_name"), c.remoteIP())