health-setup-checks: the certificate's name, Postfix's greeting, the proxy

Decided by the owner on 2026-10-13 (in place of preflight), taken up on
2026-10-16; mockups b2dcefc, cc33890, 0feaa55 (his: the greeting on a row
of its own, the name check a tag beside Names).

- The certificate: health.Certificate keeps its names (DNS names, or the
  CN without any) and ForHost holds it to SELFPOST_HOSTNAME as a TLS
  client does - VerifyHostname over the DNS names, the CN only without
  any, as OpenSSL's X509_check_host and so a sending Postfix still take it
  (src/tls/tls_client.c:439, 513). A certificate for another name fails;
  the tag beside Names says matches / not for the hostname.
- The greeting: postconf -x -h smtpd_banner, read once at the panel's
  start like the served certificate's path (main.cf is not rewritten
  after), is what smtpd writes after "220 " (src/smtpd/smtpd.c:5979). A
  first word other than the hostname warns, with what to set. A fact may
  now take the whole row (Fact.Wide, sp-wide) and carry a tag beside its
  label (Fact.LabelTag); both on the kit page.
- The proxy: auth.Module.Seen says the address the panel counts sign-ins
  against, the peer, and whether X-Forwarded-For was read or left unread
  for an untrusted peer; the Configuration card shows "Your address" and,
  in the second case, warns that every visitor shares one sign-in limit
  and names the /32 (or /128) to put in TRUSTED_PROXY_CIDR.

Tests: TestCertificateForHost (CN-only, DNS names, a wildcard, the CN
ignored beside DNS names), TestSeenAddress, TestHealthSaysWhereItSeesTheViewer,
TestHealthGreetingAndCertificateName; e2e reads the stand's right state on
Health (matches, "220 mail.e2e.test ESMTP", a direct client). go vet + go
test (26 packages) and make e2e on the test server for this tree.

Docs: help (word for word with the mockup), guide § Overview and Health,
CHANGELOG [Unreleased] § Added, roadmap row done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/panel/httpserver.go

@@ -112,6 +112,7 @@ func newPanel(cfg config, st *store.Store) (*web.Server, error) {
 		TrustedProxyCIDRs:          cfg.trustedProxies,
 		TLSCertFile:                cfg.tlsCertFile,
 		TLSServedCertFile:          cfg.tlsServedCertFile,
+		SMTPBanner:                 cfg.smtpBanner,
 		TLSReloadInterval:          cfg.tlsReloadInterval,
 		SignSocket:                 cfg.signSocket,
 		SignLocalSocket:            cfg.signLocalSocket,

cmd/panel/main.go

@@ -97,6 +97,9 @@ type config struct {
 	// how often build/postfix-cert-reload.sh looks for a renewal.
 	tlsServedCertFile string
 	tlsReloadInterval time.Duration
+	// smtpBanner is Postfix's smtpd_banner, expanded, read from postconf at
+	// the start: Health shows the greeting it makes.
+	smtpBanner string
 
 	dkimDir         string
 	dkimSelectorDef string
@@ -333,6 +336,7 @@ func run() error {
 	// postfix-config.sh has written main.cf before supervisord started the
 	// panel (build/entrypoint.sh), and nothing rewrites it after.
 	cfg.tlsServedCertFile = health.ServedCertPath()
+	cfg.smtpBanner = health.SMTPBanner()
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()