postfix: ETRN refused and not offered (audit S6)

Security audit 2026-10-17, finding S6 (low), stage 0 - proven from
outside: `ETRN <an inbound domain>` on port 25 answered "250 Queuing
started". smtpd_etrn_restrictions was empty (Postfix allows every client)
and fast_flush_domains = $relay_domains, so a stranger could flush the
deferred queue toward an inbound domain's upstream at will. Now
smtpd_etrn_restrictions=reject (etrn_cmd -> smtpd_check_etrn -> reject_code
554, Postfix 3.10.13 src/smtpd/smtpd.c) and smtpd_discard_ehlo_keywords=etrn.

e2e: testInboundRelay sends EHLO + ETRN for the inbound domain on port 25
and expects 554 and no ETRN in the EHLO reply (etrnPort25);
checkPostfixHardening reads both settings back. To be run on the server.

Docs: architecture.md § Postfix; CHANGELOG; plan.

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

CHANGELOG.md

@@ -23,6 +23,8 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   hundred long lines were megabytes held per connection.
 - **A link's token stays out of the duplicate-cookie log line** (audit A4),
   as it already did out of the origin check's.
+- **ETRN is refused and no longer offered** (audit S6): Postfix's default
+  let anyone on port 25 flush the queue toward an inbound domain's upstream.
 - **The entrypoint follows no link at the DMARC maps** (audit I2): the
   three `dmarc_*` maps root creates at the start were missing from the list
   of paths where a symbolic link is removed first.