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>

internal/web/view/outbound_test.go

@@ -434,7 +434,7 @@ func TestOutDomainDeleteNamesWhatGoesWithIt(t *testing.T) {
 		`<h1 class="title is-3">Delete example.org</h1>`, `<div class="box sp-danger-zone">`, `<h2>This cannot be undone</h2>`,
 		`Deleting <strong>example.org</strong> will:`, `permanently delete its DKIM signing key;`,
 		`delete <strong>both applications</strong> (<span class="sp-mono">prod-server</span>, <span class="sp-mono">alerts</span>) with their credentials and sender bindings;`,
-		`reload OpenDKIM so the domain is no longer signed.`, `Inbound domains are not touched.`,
+		`stop signing mail as the domain.`, `Inbound domains are not touched.`,
 		`<form method="post" action="/outbound/domains/1/delete">`, `class="button is-danger"`, `Delete example.org</button>`,
 		`href="/outbound/domains/1">Keep it</a>`,
 		`<h2>Moving it instead?</h2>`, `<a href="/outbound/domains/1/settings">Export the domain</a>`,
@@ -443,7 +443,7 @@ func TestOutDomainDeleteNamesWhatGoesWithIt(t *testing.T) {
 	// No applications: nothing to say about them.
 	out = renderSignedIn(t, "out-domain-delete", NewOutDomainDelete(admin(), 1, "example.org", nil))
 	pageLacks(t, "Delete", out, `applications`, `credentials`)
-	pageHas(t, "Delete", out, `permanently delete its DKIM signing key;`, `reload OpenDKIM`)
+	pageHas(t, "Delete", out, `permanently delete its DKIM signing key;`, `stop signing mail as the domain.`)
 	out = renderSignedIn(t, "out-domain-delete", NewOutDomainDelete(admin(), 1, "example.org", []string{"only"}))
 	pageHas(t, "Delete", out, `<strong>its application</strong> (<span class="sp-mono">only</span>)`)
 }