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/security-audit-2026-10-17.md

@@ -737,11 +737,14 @@ the argument.
   into the store (`countInboundWrites`, both writers),
   `InsertInboundSpamMessage`; TestInboundRecipientRepeatedIsOneRow,
   TestInboundJournalIsCutWhicheverWriterWrites.
-- [ ] **W1.** Refuse a custom report address on the hostname; exact hosted
+- [x] **W1.** Refuse a custom report address on the hostname; exact hosted
   address only in `AllowedRecipients`; a tag required for every recipient on
   the hostname in `checkReportDomain`; the existing test inverted; the
   accepted-risk text corrected. Check the live registry for a custom address
-  on the hostname first (none expected). — **Opus**, 1.5 h.
+  on the hostname first (none expected). — **Opus**, 1.5 h. *Done
+  2026-10-17:* the live registry holds two hosted addresses and no custom
+  one on the hostname; TestCustomReportAddressOnTheHostnameIsRefused, the
+  untagged cases in TestIngestMessageRejectsTagMismatch.
 - [ ] **A5/W2.** Delete a domain's reports in `domain.Service.Delete` (the
   cheap form); security.md § Roles says reports go with the domain. —
   **Sonnet**, 0.5 h. (The `domain_id` form is better and 2 h; take it if