panel: server status page, per-domain DNS checks, /domains move

Phase 13. Two new packages and one new screen.

internal/health owns the shared status vocabulary (ok/warn/error/unknown)
and the local checks: supervisord's process table, TLS certificate expiry
and the two milter sockets. Each check reports a problem as a status rather
than an error, so one broken component costs a line and not the page.

internal/dnscheck does the read-only lookups: forward-confirmed reverse DNS
for SELFPOST_HOSTNAME, and per-domain DKIM (compared against the key this
server actually signs with), SPF and DMARC. Every check is bounded by a
timeout and cached, and the resolver sits behind an interface so the tests
drive every branch without touching the network. The SPF check is
deliberately shallow: it looks for a mechanism literally covering the
server's address and does not follow include:/redirect=, so a record that
authorises us through an include is reported as "cannot tell" rather than
as a failure.

/status renders both, with the local checks in an HTMX-polled fragment and
the DNS lookups behind a Re-check button, and becomes the panel's landing
page: / now redirects there and the domain list lives at /domains. The
Reload button moves onto /status, where it reads as what it is — a
drift-recovery for the daemons — with text explaining what it regenerates.
A template test fails on any remaining href="/" so a stale link cannot
silently land on the wrong screen.

Also fixes a defect this made visible: the panel could never read the mail
queue in the documented deployment. postqueue relies on its setgid-postdrop
bit, which the compose file's no-new-privileges disables, so the Queue
screen always said "Could not read the mail queue" — including in the
released 1.0.0 image. The panel user is now a real member of postdrop,
which needs no setgid transition.

Verified in a container on the dev server against real DNS: PTR matching
(selfpost.mixfed.ru) and not matching (mixfed.ru), DKIM absent and
mismatched, SPF absent and via include:, DMARC p=quarantine/p=reject/absent,
and a resolver timeout degrading to "unknown" without hanging the page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/health/health.go

@@ -0,0 +1,53 @@
+// Package health reports the running container's own operating state for the
+// panel's status screen: the supervised processes, the TLS certificate Postfix
+// serves, and the milter sockets delivery depends on.
+//
+// Every check is read-only and reports a problem as a Status value rather than
+// an error return, so one broken component degrades a single line of the status
+// page instead of blanking the whole thing. The package also owns the Status
+// vocabulary shared with internal/dnscheck, so the panel renders every check —
+// local or DNS — through one set of badges.
+package health
+
+// Status is the outcome of a single check, in the order the status page treats
+// them: unknown < ok < warn < error, worst wins for a group.
+type Status string
+
+const (
+	// StatusUnknown means the check could not be performed at all (a missing
+	// setting, an unreachable resolver) — not evidence of a problem.
+	StatusUnknown Status = "unknown"
+	// StatusOK means the checked component is in its expected state.
+	StatusOK Status = "ok"
+	// StatusWarn means something is off but mail still flows.
+	StatusWarn Status = "warn"
+	// StatusError means mail delivery is (or soon will be) affected.
+	StatusError Status = "error"
+)
+
+// severity orders statuses so a group can report its worst member.
+func (s Status) severity() int {
+	switch s {
+	case StatusError:
+		return 3
+	case StatusWarn:
+		return 2
+	case StatusOK:
+		return 1
+	default:
+		return 0
+	}
+}
+
+// Worst returns the most severe of the given statuses, or StatusUnknown when
+// there are none. It is how the status page rolls a list of checks up into one
+// headline.
+func Worst(statuses ...Status) Status {
+	worst := StatusUnknown
+	for _, s := range statuses {
+		if s.severity() > worst.severity() {
+			worst = s
+		}
+	}
+	return worst
+}