upstream-tls-verify: Verified TLS to an inbound domain's upstream

Owner, 2026-10-18: "Оба предложения делаем"; the mockup is c10cd78. The
security audit's M7: Required is Postfix's encrypt level, which checks no
certificate - it keeps the mail from a listener, not from one who answers
in the upstream's place.

- A fourth TLS mode, Verified, stored as Postfix's level secure (migration
  0014 rebuilds inbound_transports with the wider CHECK; schema 14) and
  written to the TLS policy map like the others. Postfix matches the
  certificate against the next hop's name (smtp_tls_secure_cert_match =
  nexthop, dot-nexthop, the default; postconf(5)), so Verified is refused
  for an upstream given by address (checkSecureHost).
- Postfix's SMTP client trusts Debian's ca-certificates bundle
  (smtp_tls_CAfile): Postfix trusts none by default (DEF_SMTP_TLS_CA_FILE
  "", DEF_TLS_APPEND_DEF_CA 0). The levels may and encrypt only log the
  verdict, so mail to recipients' MX is unchanged.
- The page: the select's fourth option, a help line under it, the lead
  "over verified TLS", the list's tag "verified"; Help word for word with
  the mockup; guide § Inbound relay, with a posttls-finger command to try
  an upstream first.

Tests: TestVerifiedTLSNeedsTheUpstreamsName, TestRenderInboundMapsSecure,
TestUpstreamTLSModeSecure; schema tests at 14. e2e
inbound_upstream_verified_tls: an address refused, a name saved as
"[sink]:25 secure" in tls_policy, the page says verified, the CA file set
and present, and posttls-finger -l secure with it calls the stand's
self-signed port 25 "Untrusted" (the stand has no TLS upstream). make e2e
(65 passed) and go vet + go test (26 packages) on the test server for this
tree. Tried against the live upstream from the 2.14.5 container:
mc-ent.mixfed.ru gives "Verified TLS connection established".

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

CHANGELOG.md

@@ -7,6 +7,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Added
 
+- **Verified TLS to an inbound domain's upstream** (upstream-tls-verify; the
+  security audit's M7): a fourth choice beside Opportunistic, Required and
+  Off - Postfix's `secure` level, which also checks that the upstream's
+  certificate was issued by a trusted authority for its name. The upstream
+  is then given by name. Postfix's SMTP client now trusts Debian's
+  `ca-certificates`; mail to the recipients' MX is unchanged. Migration
+  0014 widens the stored TLS mode; schema 14.
 - **A DMARC record for the server's own name** (server-dmarc-hint; the
   security audit's S3): Server › Settings › Notification mail gives
   `v=DMARC1; p=reject` at `_dmarc.<hostname>` beside the DKIM and SPF