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>

README.md

@@ -88,6 +88,16 @@ mail lands in spam even though SelfPost delivered it correctly — DKIM passing
 doesn't help if SPF/DMARC are absent. **Whenever you add a new domain in the
 panel, add its DNS records at the same time**, not later.
 
+The panel checks both scopes for you and tells you what is actually published:
+the *Status* page verifies the server's hostname and its reverse record
+(forward-confirmed reverse DNS), and each domain's page shows a *DNS status*
+card comparing the published DKIM record against the key this server signs with,
+plus the domain's SPF and DMARC records. Results are cached for a few minutes;
+use *Re-check* right after publishing a record. The SPF check is deliberately
+shallow — it looks for a mechanism that literally covers this server's address
+and does not follow `include:` or `redirect=`, so a record that authorizes the
+server through an include is reported as "cannot tell" rather than as a failure.
+
 ## IP warmup
 
 A brand-new IP has no sending history, so receiving servers are cautious with