Phase 5: full outbound Postfix relay (465/587 SASL+TLS, sender binding, milters)

Generate the relay config from the environment at container start
(build/postfix-config.sh, run from entrypoint.sh):

- smtps 465 (implicit TLS) primary + optional submission 587 (STARTTLS),
  chroot=n so smtpd reaches the sasldb2/sender map under /data.
- Cyrus SASL against the panel-maintained sasldb2; realm left implicit so the
  authenticated name equals the bare login in smtpd_sender_login_maps.
- reject_sender_login_mismatch + relay/recipient restrictions with no
  permit_mynetworks: credentials-only, open relay impossible (spec 5, 5.1).
- TLS cert/key from TLS_CERT_FILE/TLS_KEY_FILE; daily postfix reload picks up
  renewed certs (postfix-cert-reload.sh under supervisord, spec 5.2).
- anvil level-1 rate limit from env (spec 5 p.5).
- Milter chain with per-milter action: OpenDKIM strict (tempfail), journal
  fail-open (accept) so monitoring never blocks the relay (spec 7.3).

Two integration fixes found on the server:
- postconf -F '*/*/chroot=n': Debian's chrooted delivery agent can't read
  /etc/resolv.conf, so MX lookups failed and mail never left.
- entrypoint sets /run/opendkim and /run/selfpost to group selfpost + setgid,
  and the journal stub chmods its socket 0660, so postfix can connect to both
  milter sockets (strict OpenDKIM was milter-rejecting all mail otherwise).

Verified on selfpost.example.com: gofmt/vet/test green, image builds; container
e2e — 465 auth+send DKIM-signed (d=domain,s=selfpost), 587 STARTTLS auth,
cross-domain sender 553, list-mode per-address binding, unauth relay 554,
real outbound delivery reaching the recipient MX over TLS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/panel/journalstub.go

@@ -27,6 +27,13 @@ func serveJournalStub(ctx context.Context, socketPath string) error {
 	if err != nil {
 		return err
 	}
+	// Postfix (user `postfix`) connects to this socket as a milter and needs
+	// group access. The socket inherits group `selfpost` from the setgid parent
+	// dir entrypoint.sh prepares; make it group read/write so postfix can reach
+	// it (connecting to a Unix socket needs write permission on the node).
+	if err := os.Chmod(socketPath, 0o660); err != nil {
+		return err
+	}
 
 	// Closing the listener unblocks Accept and unlinks the socket file.
 	go func() {