Self-host Mailu mail stack at mail.profullstack.com

Members get a real <name>@mail.profullstack.com mailbox, served by a
co-located Mailu (Postfix+Dovecot+Roundcube+rspamd) Docker stack. Coexists
with the host Caddy: Mailu owns the mail ports; Caddy fronts the loopback
webmail and supplies the TLS cert (TLS_FLAVOR=mail), the same cert-copy
pattern as the Ergo/IRC and NNTP services.

- deploy/mailu/: docker-compose.yml, mailu.env.example, refresh-certs.sh
  (copy Caddy's mail cert into Mailu on renewal), provision-mailbox.sh
  (member mailbox + Dovecot gateway master user), README.
- setup.sh: MAIL flag + mail.${DOMAIN#*.} Caddy site + §9e (cert timer,
  mail-port firewall, conditional compose bring-up, AGENTBBS_MAIL_* env).
- docs/mail.md: architecture, DNS (MX/SPF/DKIM/DMARC/PTR), gateway
  master-user setup, env, provisioning, webmail-only policy.

Apex profullstack.com stays corporate; member mail is only on mail.*.
Infra is inspection-verified (bash -n, YAML lint); deploy pending.

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

deploy/mailu/.gitignore

@@ -0,0 +1,3 @@
+mailu.env
+certs/
+data/