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.example.com) and not matching (example.com), 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/templates/layout.html

@@ -92,6 +92,24 @@
   @media (prefers-color-scheme: dark) {
     .nav .links [aria-current] { color: #e6e8eb !important; background: #22303f !important; }
   }
+  /* Status badges: one vocabulary (ok/warn/error/unknown) shared by the server
+     status page and the per-domain DNS checks, so a colour means the same thing
+     everywhere. The class suffix is the check's own status value. */
+  .st {
+    display: inline-block; padding: 0.05rem 0.45rem; border-radius: 999px;
+    font-size: 0.7rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.04em;
+    vertical-align: middle; border: 1px solid transparent;
+  }
+  .st-ok { background: #ecfdf3; color: #067647; border-color: #abefc6; }
+  .st-warn { background: #fffaeb; color: #b54708; border-color: #fedf89; }
+  .st-error { background: #fef3f2; color: #b42318; border-color: #fecdca; }
+  .st-unknown { background: #f0f2f4; color: #6b7280; border-color: #e2e5e9; }
+  @media (prefers-color-scheme: dark) {
+    .st-ok { background: #0d2818 !important; color: #75d99b !important; border-color: #1a5336 !important; }
+    .st-warn { background: #2e2308 !important; color: #f5c86b !important; border-color: #6b5210 !important; }
+    .st-error { background: #2d1211 !important; color: #f5a29b !important; border-color: #6b201a !important; }
+    .st-unknown { background: #22262b !important; color: #9aa3ad !important; border-color: #2b3138 !important; }
+  }
   .code-row { display: flex; align-items: flex-start; gap: 0.5rem; }
   .code-row .code { flex: 1; min-width: 0; }
   button.copy {
@@ -121,7 +139,8 @@
 {{define "nav"}}
 <nav class="nav">
   <div class="links">
-    {{if eq .Active "domains"}}<span aria-current="page">Domains</span>{{else}}<a href="/">Domains</a>{{end}}
+    {{if eq .Active "status"}}<span aria-current="page">Status</span>{{else}}<a href="/status">Status</a>{{end}}
+    {{if eq .Active "domains"}}<span aria-current="page">Domains</span>{{else}}<a href="/domains">Domains</a>{{end}}
     {{if eq .Active "sendlog"}}<span aria-current="page">Send log</span>{{else}}<a href="/sendlog">Send log</a>{{end}}
     {{if eq .Active "queue"}}<span aria-current="page">Queue</span>{{else}}<a href="/queue">Queue</a>{{end}}
     {{if eq .Active "logtail"}}<span aria-current="page">Log</span>{{else}}<a href="/logtail">Log</a>{{end}}