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/web/handlers_status.go

@@ -0,0 +1,131 @@
+package web
+
+import (
+	"net/http"
+	"strings"
+
+	"codeberg.org/mix/selfpost/internal/health"
+)
+
+// handleStatus renders the server status page: the panel's landing page and the
+// one screen that answers "is the service healthy and will mail be accepted"
+// (phase 13.A). The cheap local checks live in the polled "status_body"
+// fragment; the hostname/PTR lookup and the configuration reload sit outside it,
+// because neither belongs on a five-second timer.
+func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
+	data := s.statusBody()
+	srv := s.dns.Server(s.cfg.Hostname, false)
+
+	data["Title"] = "SelfPost — status"
+	data["User"] = currentUser(r)
+	data["Active"] = "status"
+	data["Flash"] = statusFlash(r)
+	data["Hostname"] = s.cfg.Hostname
+	data["PTR"] = srv.PTR
+	s.render(w, http.StatusOK, "status", data)
+}
+
+// handleStatusFragment serves the HTMX polling fragment for the local checks
+// (spec 7.1: fragment endpoints return HTML, not JSON).
+func (s *Server) handleStatusFragment(w http.ResponseWriter, _ *http.Request) {
+	s.renderFragment(w, http.StatusOK, "status_body", s.statusBody())
+}
+
+// handleStatusRecheck forces a fresh hostname/PTR lookup, bypassing the cache,
+// and returns to the page. DNS is the one part of this screen that talks to the
+// network, so it refreshes on demand rather than with the poll.
+func (s *Server) handleStatusRecheck(w http.ResponseWriter, r *http.Request) {
+	s.dns.Server(s.cfg.Hostname, true)
+	http.Redirect(w, r, "/status?rechecked=1", http.StatusSeeOther)
+}
+
+// statusBody collects the four local checks the fragment renders. Each one
+// reports its own problem rather than failing the page, so a broken component
+// costs one line and not the whole screen.
+func (s *Server) statusBody() map[string]any {
+	procs, procErr := health.Processes()
+	procStatus := health.StatusUnknown
+	if procErr != nil {
+		// Outside the container (or if the control socket is gone) there is
+		// nothing to report — "unknown", not "everything is broken".
+		logf("panel: status: supervisorctl: %v", procErr)
+	} else {
+		for _, p := range procs {
+			procStatus = health.Worst(procStatus, p.Status)
+		}
+	}
+
+	queueText, queueErr := readQueue()
+	queueStatus := health.StatusOK
+	if queueErr != "" {
+		queueStatus = health.StatusWarn
+	}
+
+	cert := health.CheckCertificate(s.cfg.TLSCertFile)
+	sockets := []health.Socket{
+		// OpenDKIM signs every outgoing message and Postfix is configured to
+		// tempfail without it: a missing socket stops mail.
+		health.CheckSocket("OpenDKIM", s.cfg.OpenDKIMSocket, true),
+		// The journal-milter only records the send log and fails open.
+		health.CheckSocket("send-log", s.cfg.JournalSocket, false),
+	}
+	socketStatus := health.StatusUnknown
+	for _, sock := range sockets {
+		socketStatus = health.Worst(socketStatus, sock.Status)
+	}
+
+	overall := health.Worst(procStatus, queueStatus, cert.Status, socketStatus)
+	return map[string]any{
+		"Processes":      procs,
+		"ProcessError":   procErr != nil,
+		"ProcessStatus":  procStatus,
+		"QueueSummary":   queueSummary(queueText),
+		"QueueError":     queueErr,
+		"QueueStatus":    queueStatus,
+		"Cert":           cert,
+		"Sockets":        sockets,
+		"SocketStatus":   socketStatus,
+		"OverallStatus":  overall,
+		"OverallHeading": overallHeading(overall),
+	}
+}
+
+// queueSummary reduces postqueue's listing to the one line worth showing on the
+// status page; the full listing has its own screen (spec 7.2.11). postqueue
+// prints either "Mail queue is empty" or a trailing "-- N Kbytes in M Requests."
+func queueSummary(out string) string {
+	lines := strings.Split(strings.TrimSpace(out), "\n")
+	for i := len(lines) - 1; i >= 0; i-- {
+		if line := strings.TrimSpace(lines[i]); line != "" {
+			return strings.TrimSpace(strings.TrimPrefix(line, "--"))
+		}
+	}
+	return ""
+}
+
+// overallHeading turns the worst check into the page's one-line verdict.
+func overallHeading(worst health.Status) string {
+	switch worst {
+	case health.StatusError:
+		return "A component needs attention — see the details below."
+	case health.StatusWarn:
+		return "Running, with warnings below."
+	case health.StatusOK:
+		return "All components are running normally."
+	default:
+		return "Some checks could not be performed."
+	}
+}
+
+// statusFlash maps a fixed redirect flag to a fixed message, so status text
+// after a redirect is never attacker-influenced.
+func statusFlash(r *http.Request) string {
+	switch {
+	case r.URL.Query().Get("reloaded") != "":
+		return "Configuration regenerated from the database; OpenDKIM and Postfix have re-read it."
+	case r.URL.Query().Get("rechecked") != "":
+		return "DNS re-checked."
+	default:
+		return ""
+	}
+}