panel-dkim-signing stage 2: OpenDKIM removed

The panel's signer is the only one. Gone: the opendkim package and its
configuration, the syslog-relay and opendkim programs, /dev/log and
/run/opendkim, the reload by USR1 through supervisord
(internal/supervisor/supervisor.go), internal/syslogrelay, the port of
OpenDKIM's From: parser (internal/milter/mailparse.go and its probe,
harness and test/upstream scripts), OPENDKIM_SOCKET and OPENDKIM_DIR.

Keys: internal/domain/keys.go (Keys, NewKeys, DKIM_DIR=/data/dkim) keeps
the keys and writes a selector table, /data/dkim/selectors ("<name>
<selector>"), which the signer reads in place of the KeyTable - it reads
no database, as the sender check reads none. The table is written before
the signer serves (EnsureServerSigned moved there from the notifier).
Keys and table 0600, directories 0700, the panel's. The entrypoint copies
/data/opendkim/keys once, as the panel user, into /data/dkim.new renamed
when whole, and stops the start if the copy fails; /data/opendkim is left
for a rollback to 2.7.x.

Sender check: one RFC 5322 address as a mail program reads it, held
against the bindings; the 256-byte refusal and the texts naming OpenDKIM
go. Allowed() folded the domain through strings.ToLower, which makes the
Kelvin sign a "k" - the comparison with OpenDKIM's reading had hidden it;
it folds ASCII only now.

Health: two dkim-signer rows (applications, notifications) in Milter
sockets, opendkim out of the liveness set; the card no longer takes any
row with a note for the spam filter. Texts as in the design commit.

KILL stays in deploy/docker-compose.yml: measured on the test server,
root without it cannot signal a process of another uid - supervisord
stopping the panel, Postfix's master its children. The plan's "only use
was the reload" was wrong; the comment now gives the real uses.

e2e: one signature, the panel's, verified by dkimpy; the /data/dkim modes;
the OpenDKIM log and process checks, testNeverSignedForAnother and the
cited opendkim, libopendkim11 and libmilter1.0.1 go.

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

cmd/panel/main.go

@@ -28,7 +28,6 @@ import (
 	"github.com/mixeme/selfpost/internal/inbound"
 	"github.com/mixeme/selfpost/internal/logtail"
 	"github.com/mixeme/selfpost/internal/store"
-	"github.com/mixeme/selfpost/internal/syslogrelay"
 )
 
 func main() {
@@ -48,21 +47,12 @@ func main() {
 	}
 
 	showVersion := flag.Bool("version", false, "print version and exit")
-	relay := flag.String("syslog-relay", "", "relay syslog(3) datagrams sent to this socket path to stdout, and do nothing else")
 	flag.Parse()
 
 	if *showVersion {
 		fmt.Println(buildinfo.Version)
 		return
 	}
-	if *relay != "" {
-		// The container's /dev/log, for OpenDKIM (internal/syslogrelay;
-		// build/supervisord.conf, [program:syslog-relay]).
-		if err := syslogrelay.Run(*relay, os.Stdout); err != nil {
-			log.Fatal(err)
-		}
-		return
-	}
 
 	log.SetFlags(log.LstdFlags | log.LUTC)
 	log.SetPrefix("panel: ")
@@ -98,19 +88,17 @@ type config struct {
 	rateLimitMessagesPerIP int
 	rateLimitWindowSeconds int
 
-	// Read-only inputs to the panel's status page: the certificate Postfix
-	// serves and the two milter sockets it connects to. The defaults mirror
-	// build/postfix-config.sh, so the status page checks exactly what Postfix
-	// was configured with.
-	tlsCertFile    string
-	opendkimSocket string
+	// Read-only input to the panel's status page: the certificate Postfix
+	// serves. The default mirrors build/postfix-config.sh, so the status
+	// page checks exactly what Postfix was configured with.
+	tlsCertFile string
 	// tlsServedCertFile is the certificate Postfix serves (its copy of
 	// tlsCertFile), read from postconf at the start; tlsReloadInterval is
 	// how often build/postfix-cert-reload.sh looks for a renewal.
 	tlsServedCertFile string
 	tlsReloadInterval time.Duration
 
-	opendkimDir     string
+	dkimDir         string
 	dkimSelectorDef string
 
 	saslDBPath string
@@ -184,11 +172,10 @@ func loadConfig() config {
 
 		tlsCertFile:       envDefault("TLS_CERT_FILE", "/etc/postfix/tls/fullchain.pem"),
 		tlsReloadInterval: time.Duration(envInt("TLS_RELOAD_INTERVAL_SECONDS", 86400)) * time.Second,
-		opendkimSocket:    envDefault("OPENDKIM_SOCKET", "/run/opendkim/opendkim.sock"),
 
-		// Per-domain DKIM state (architecture.md § OpenDKIM). The directory layout
-		// matches what entrypoint.sh prepares (setgid, shared `selfpost` group).
-		opendkimDir:     envDefault("OPENDKIM_DIR", filepath.Join(dataDir, "opendkim")),
+		// Per-domain DKIM state (architecture.md § DKIM signing): the keys and
+		// the selector table, the layout entrypoint.sh prepares.
+		dkimDir:         envDefault("DKIM_DIR", filepath.Join(dataDir, "dkim")),
 		dkimSelectorDef: envDefault("DKIM_SELECTOR_DEFAULT", "selfpost"),
 
 		// Application SASL accounts and the Postfix sender map (architecture.md §
@@ -419,7 +406,7 @@ func run() error {
 		{"http", func(ctx context.Context) error { return serveHTTP(ctx, cfg, st) }},
 		{"journal-milter", func(ctx context.Context) error { return serveJournal(ctx, cfg, st) }},
 		{"sender-check", func(ctx context.Context) error { return serveSenderCheck(ctx, cfg, st) }},
-		{"dkim-signer", func(ctx context.Context) error { return serveSigner(ctx, cfg) }},
+		{"dkim-signer", func(ctx context.Context) error { return serveSigner(ctx, cfg, st) }},
 		{"log-tailer", func(ctx context.Context) error {
 			return logtail.Run(ctx, cfg.mailLog, st, func() int {
 				days, err := st.GetSendLogRetentionDays(cfg.retentionDays)