milter: the level-2 domain limit is the login's domain's (audit M1)

Security audit 2026-10-17, finding M1, stage 1. overLimit took
domainOf(MAIL FROM) whatever the login, and MailFrom ran it with an empty
login: with smtpd_delay_reject at its default Postfix runs no sender
restriction at MAIL and sends the milter's MAIL event at once, so a
stranger on 465/587 with no AUTH, or another tenant's application, could
name a victim domain in MAIL FROM and take its slots for the 10-minute
in-flight TTL, get the victim's application answered 451, and have a limit
row (and the "a rate limit is refusing mail" notification) written under
the victim's domain.

Now overLimit returns at once without a login and takes the domain from
ApplicationByLogin (loginDomain, fail-open like the rest); recordRejected's
Domain is the login's too. The send log already counts a domain's messages
by the login's domain (store.CountMessages, loginDomainID), so the limit
and the count now agree. The null-sender special case is gone: every
message is held to its login's domain.

Tests: TestRateLimitIsNotReservedByAnotherSendersMailFrom (a stranger with
no login; another tenant's application) - fails on the old code at the
second MAIL FROM; the domain-limit tests sign in as app1 at example.com.
e2e domain_limit_not_reserved_by_a_stranger: a domain with a limit of one,
three unauthenticated MAIL FROM naming it on 465, then its own message
passes and its second is refused 451.

Docs: security.md § Mail path; CHANGELOG; plan.

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

test/e2e/main_test.go

@@ -315,6 +315,9 @@ func TestE2E(t *testing.T) {
 	run("rate_limit_refusal_is_notified", func(t *testing.T) {
 		testRateLimitRefusalIsNotified(t, sc)
 	})
+	run("domain_limit_not_reserved_by_a_stranger", func(t *testing.T) {
+		testDomainLimitNotReservedByAStranger(t, sc)
+	})
 	run("negative_sender_login_mismatch", func(t *testing.T) {
 		testSenderLoginMismatch(t, sc)
 	})