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>

build/supervisord.conf

@@ -88,6 +88,21 @@ stdout_logfile_maxbytes=0
 stderr_logfile=/dev/stderr
 stderr_logfile_maxbytes=0
 
+; Periodic TLS-certificate refresh (spec 5.2 p.4). Runs a daily `postfix reload`
+; so certificates the reverse-proxy renews in the read-only mount are picked up.
+; Long-running (it loops), runs as root so it can reload Postfix, and never exits
+; non-zero, so it neither trips the crashexit listener nor needs restarting.
+[program:cert-reload]
+command=/usr/local/bin/postfix-cert-reload.sh
+priority=400
+autostart=true
+autorestart=true
+startsecs=0
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+
 [eventlistener:crashexit]
 command=/usr/local/bin/crashexit.py
 events=PROCESS_STATE_FATAL