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/migrate/firewall.go

@@ -0,0 +1,41 @@
+package migrate
+
+import (
+	"os/exec"
+	"strconv"
+	"strings"
+)
+
+// ufw — вызов ufw (в тестах подменяется). Нет ufw — ошибка, и правила не трогаются.
+var ufw = func(args ...string) ([]byte, error) {
+	if _, err := exec.LookPath("ufw"); err != nil {
+		return nil, err
+	}
+	return exec.Command("ufw", args...).CombinedOutput()
+}
+
+// OpenFirewall — открыть в UFW порты, на которые панель переезжает при приёме: подписка и хаб
+// берут порты старого сервера, а install.sh открыл только те, что были при установке. Без этого
+// нода #0 не дозвонится до хаба, а ретранслятор — до подписки. UFW не установлен или не активен —
+// ничего не делаем (как install.sh). opened — что открыто, failed — что не удалось.
+func OpenFirewall(ports ...int) (opened, failed []string) {
+	out, err := ufw("status")
+	if err != nil || !strings.Contains(strings.ToLower(string(out)), "status: active") {
+		return nil, nil
+	}
+	for _, p := range ports {
+		if p <= 0 {
+			continue
+		}
+		rule := strconv.Itoa(p) + "/tcp"
+		if _, err := ufw("allow", rule); err != nil {
+			failed = append(failed, rule)
+		} else {
+			opened = append(opened, rule)
+		}
+	}
+	if len(opened) > 0 {
+		ufw("reload")
+	}
+	return opened, failed
+}