panel-notifications: the notifier, its mail and the server's own DKIM key

internal/notify applies the owner's rules. Each subject has a row of state,
so a thing is said once and a restart says nothing again:

- DNS of a sending domain: said after two hourly checks in a row found a
  record wrong, said to be over after two found it right. A lookup that did
  not answer moves nothing; a warning (no DMARC record) is not a subject.
- TLS certificate: at 14 days and at 3, per certificate.
- Queue: a message older than three days, once; once more when none is.
- Level-2 limit: from the rejected rows of the send log, every five minutes;
  at most once a day per domain and application.

Global users are told everything, domain users about their domains; one
message per recipient. With the instance switch off nothing is checked and no
state moves. A message that could not be handed to Postfix is tried again at
the next check.

Mail leaves by sendmail(1) as postmaster@<hostname>, addresses after "--",
no shell. For it to be believed the server's own name is signed like a
sending domain: domain.Service keeps a key for it and a line in OpenDKIM's
tables without a registry row, and non_smtpd_milters now names OpenDKIM. A
sending domain of the same name shares its key with the server.
dnscheck.ServerMail checks the name's DKIM and SPF records;
postfix.QueueMessages reads postqueue -j for arrival times.

The notifier is a sixth role of the panel process. Nothing in the panel
shows or switches it yet.

Checked on Windows: go vet clean; go test passes for notify, store, postfix,
dnscheck, cmd, and domain apart from TestWriteLoadPrivateKeyRoundtrip, which
fails there on file modes and always did. Not yet run in a container: whether
the unprivileged panel may submit is for the e2e step.

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

internal/postfix/queuejson_test.go

@@ -0,0 +1,43 @@
+package postfix
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestParseQueueJSON(t *testing.T) {
+	out := []byte(`{"queue_name": "deferred", "queue_id": "41E862C00D9E", "arrival_time": 1791000000, "message_size": 446, "forced_expire": false, "sender": "app@example.com", "recipients": [{"address": "a@example.net", "delay_reason": "connect to mx.example.net[203.0.113.9]:25: Connection timed out"}]}
+{"queue_name": "active", "queue_id": "5F00AA12", "arrival_time": 1791250000, "message_size": 900, "sender": "", "recipients": [{"address": "b@example.net"}]}
+
+not json at all
+{"queue_name": "deferred", "queue_id": "", "arrival_time": 1791250000}
+{"queue_name": "deferred", "queue_id": "6C00BB34", "arrival_time": 17912`)
+
+	got := parseQueueJSON(out)
+	if len(got) != 2 {
+		t.Fatalf("parsed %d messages, want 2: %+v", len(got), got)
+	}
+	if got[0].ID != "41E862C00D9E" || got[0].Queue != "deferred" || got[0].Sender != "app@example.com" ||
+		!got[0].Arrival.Equal(time.Unix(1791000000, 0)) {
+		t.Errorf("first message = %+v", got[0])
+	}
+	if got[1].ID != "5F00AA12" || got[1].Queue != "active" {
+		t.Errorf("second message = %+v", got[1])
+	}
+	if len(parseQueueJSON(nil)) != 0 {
+		t.Error("an empty queue is not empty")
+	}
+}
+
+// A queue that cannot be listed is an error, not an empty queue: the caller
+// must not conclude that nothing is waiting.
+func TestQueueMessagesReportsAFailedListing(t *testing.T) {
+	old := readQueueJSON
+	t.Cleanup(func() { readQueueJSON = old })
+	readQueueJSON = func() ([]byte, error) { return nil, errors.New("postqueue: Permission denied") }
+
+	if msgs, err := QueueMessages(); err == nil {
+		t.Fatalf("no error for a failed listing (got %v)", msgs)
+	}
+}