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>

build/Dockerfile

@@ -67,6 +67,17 @@ RUN groupadd --system selfpost \
  && usermod -aG selfpost opendkim \
  && usermod -aG selfpost postfix
 
+# The panel reads the mail queue with `postqueue -p` (spec 7.2.11, and the
+# status page's queue card). postqueue is setgid postdrop, which normally gives
+# it the group needed to reach Postfix's showq socket — but the documented
+# deployment runs with `no-new-privileges`, which disables setgid transitions,
+# so the panel would always see "Permission denied". Making `panel` a real
+# member of postdrop grants the same access without relying on a setgid
+# escalation the hardening deliberately forbids. postdrop membership is
+# read-side only: it does not let the panel bypass any Postfix restriction that
+# a local user does not already have through the world-executable sendmail.
+RUN usermod -aG postdrop panel
+
 # Runtime directories: milter sockets and the consolidated persistent root.
 RUN mkdir -p /run/opendkim /run/selfpost /data \
  && chown opendkim:opendkim /run/opendkim \