panel-notifications: guide, architecture, security; refusals looked for every minute

guide.md gains Notifications: the four things, when each is said and said
again, who is told, the two switches, and the two DNS records of the server's
own name that make the mail arrive. architecture.md: the notifier as the
fifth role, its readings, state, recipients and sending; the server's own key
in OpenDKIM's tables; non_smtpd_milters. security.md: the panel submits mail
of its own - how, to whom, and what text from outside can reach a body.

That text - a queued message's sender, a DNS finding - is reduced to
printable ASCII and cut before it is printed. Refused mail is looked for
every minute instead of every five: it is one indexed query, and a refusal is
mail not being sent now. The image's note on the panel's postdrop membership
no longer calls it read-side only.

go test ./internal/notify passes.

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

docs/plans/panel-notifications.md

@@ -111,9 +111,9 @@ to the owner before any code ([panel-redesign.md](panel-redesign.md)
 - [x] Migration and store: user flag, instance switch, event state
 - [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)
+- [x] Level-2 refusal event (from the `rejected` rows of the send log, looked at every minute)
 - [ ] Panel: Account, Settings, Health check, help
-- [ ] guide.md, architecture.md, security.md, schema-migrations.md
+- [x] guide.md, architecture.md, security.md, schema-migrations.md
 - [ ] `go vet`, `go test`, e2e: an event reaches a mailbox once and clears once
 - [ ] Technical review
 - [ ] Security review