dmarc-ingest: what is not a report this relay keeps is dropped, not retried

Owner, 2026-10-11: foreign and malformed reports are turned away.

The command answered every failure with EX_TEMPFAIL, so Postfix kept a
message that could never be stored and offered it again for days, each
attempt adding to the parse-failure count. IngestMessage now marks those
failures (unreadable as an aggregate report, a domain not sent from
here, another domain's report address) with ErrRejected; the command
counts one failure, writes the reason where Postfix puts it on the delivery line of the mail log
and reports the delivery as done. No bounce: the envelope sender of such
a message is whatever its author wrote, and the report addresses are
public. A failing store is still EX_TEMPFAIL.

  go vet ./cmd/... ./internal/dmarc/: ok
  go test ./cmd/panel/ ./internal/dmarc/ ./internal/web/...: ok

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

docs/guide.md

@@ -893,6 +893,12 @@ from *DMARC reports* in the domain page's menu. A user with the *domain* role
 sees the reports of the domains assigned to them, without the *Ingest* box.
 Summaries are pruned (500 kept, 90 days max).
 
+A message that is not a report the relay keeps — unreadable as an aggregate
+report, about a domain not sent from here, or delivered to another domain's
+report address — is dropped without a retry and without a bounce. It adds one
+to *parse failures*, and the reason is on the message's delivery line in the mail log
+(*Server › System log*, `dropped: …`).
+
 **Not an open relay.** The inbound smtpd offers no SASL. With DMARC ingest
 alone, `check_recipient_access` permits only configured report addresses;
 everything else is rejected. With inbound relay enabled too, both allow-lists