dmarc: no custom report address on the server's own name (audit W1)

Security audit 2026-10-17, finding W1, stage 1. HandleDomainDMARC took any
address validate.Email takes; AllowedRecipients opened every stored address
on the hostname, not only the domain's hosted one; checkReportDomain
cross-checked the claimed domain only against a dmarc-reports+<tag> local
part. So a domain user of tenant A could store ops@<hostname> and from then
on anyone could mail a report claiming tenant B's domain to it, stored and
shown under B and counted toward B's 500; or store B's hosted address and
open B's mailbox without B choosing hosted. TestAllowedRecipients asserted
ops@<host> was allowed - a choice, now reversed.

Now: a custom address on the hostname is refused on the form ("choose
SelfPost hosted"); AllowedRecipients adds a domain's address only when it
is that domain's own hosted address; checkReportDomain requires a tag and
the claimed domain to equal it. The live registry (checked 2026-10-17)
holds two hosted addresses and no custom one on the hostname, so nothing
there changes.

Tests: TestCustomReportAddressOnTheHostnameIsRefused (typed, another
domain's hosted, the domain's own hosted typed as custom; an address
elsewhere still saves); TestAllowedRecipientsFollowTheDomainAddresses
inverted plus another domain's hosted address stored; untagged addresses
rejected in TestIngestMessageRejectsTagMismatch; the ingest tests deliver
to tagged addresses.

Docs: security.md § Accepted risks (DMARC), guide § DMARC reports;
CHANGELOG; plan.

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

docs/guide.md

@@ -1343,7 +1343,8 @@ settings) and nowhere else:
   server whose reports are parsed and shown under *DMARC reports*. Offered only
   while `DMARC_REPORTS_ENABLE=true`.
 - **A specific address** — any mailbox, typed for this domain; it receives the
-  raw XML. Beside the field, *Use my e-mail* fills it with the e-mail of your
+  raw XML. Not one on `SELFPOST_HOSTNAME`: to have this server receive the
+  reports, choose *SelfPost hosted*. Beside the field, *Use my e-mail* fills it with the e-mail of your
   profile (Account); it only fills the field, and saving is the form's own
   button. The button is absent when your profile has no e-mail.
 
@@ -1351,8 +1352,8 @@ Changing the choice changes the DMARC record to publish; the domain page shows
 the new value. There is no panel-wide or per-user default. Only addresses on
 `SELFPOST_HOSTNAME` that a domain has chosen are accepted on port 25 - the
 hosted address `dmarc-reports+<domain>@<hostname>` of a domain set to
-*SelfPost hosted*, or a typed address that happens to be on this host. There
-is no mailbox for the server as a whole.
+*SelfPost hosted* - and a report is kept only when it is for the domain the
+address is tagged with. There is no mailbox for the server as a whole.
 
 **DNS.** Publish the usual `_dmarc` TXT on each sending domain with
 `rua=mailto:…` pointing at the address the domain shows. Receivers deliver to the