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>

docs/plans/panel-notifications.md

@@ -109,9 +109,9 @@ to the owner before any code ([panel-redesign.md](panel-redesign.md)
 
 - [x] Mockups: Account checkbox, Settings switch and records, Health status card, help topic; accepted by the owner 2026-10-12 (`design:` commit)
 - [x] Migration and store: user flag, instance switch, event state
-- [ ] Sending: local submission from `postmaster@<hostname>`, message text
-- [ ] Background checker: DNS, certificate, queue; the rules of *When a message is sent*
-- [ ] Level-2 refusal event
+- [x] Sending: local submission from `postmaster@<hostname>`, message text (`sendmail` as the panel user; signed through `non_smtpd_milters` with a key of the server's own name; no link in the text - the panel does not know its own address)
+- [x] Background checker: DNS, certificate, queue; the rules of *When a message is sent*
+- [x] Level-2 refusal event (from the `rejected` rows of the send log, looked at every five minutes)
 - [ ] Panel: Account, Settings, Health check, help
 - [ ] guide.md, architecture.md, security.md, schema-migrations.md
 - [ ] `go vet`, `go test`, e2e: an event reaches a mailbox once and clears once