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>

cmd/panel/httpserver.go

@@ -37,6 +37,9 @@ func newMailStack(cfg config, st *store.Store) *mailStack {
 	odk := domain.NewOpenDKIM(cfg.opendkimDir)
 	apps := app.NewService(st, app.NewSASLDB(cfg.saslDBPath, cfg.saslRealm), pf)
 	domains := domain.NewService(st, odk, apps, cfg.dkimSelectorDef)
+	// The panel's notifications are sent as postmaster@ the server's own name,
+	// and signed as it.
+	domains.SetServerName(cfg.hostname)
 	inb := inbound.NewService(st, pf)
 	dmarcSvc := dmarc.NewService(st, pf, cfg.hostname, cfg.dmarcEnabled)
 	return &mailStack{