Feat/mail all members (#55)

* feat(mail): give every verified member a free @bbs.profullstack.com mailbox

Email was built but paid-only (Founding Lifetime gate) and never wired to a
running backend. Make it a free benefit of membership and split the address
domain from the mail-server host.

- internal/mailu: Mailu admin-API client; EnsureUser idempotently provisions a
  mailbox via the loopback admin REST API (token = mailu.env API_TOKEN).
- main.go: auto-provision <name>@<mailDomain> at join@ verification and on first
  Mail open; un-gate the Mail hub entry + mail@ (membership/email-verified, not
  Premium); address domain (AGENTBBS_MAIL_ADDR_DOMAIN, default the BBS host) is
  now distinct from the mail server host (AGENTBBS_MAIL_DOMAIN) and the webmail
  URL. Drop the forwardemail alias path (Mailu now owns delivery for everyone).
- mailbox: gate on membership (a registered handle) instead of Paid;
  ErrNotPaid -> ErrNotMember.
- join@ copy: list email under free membership; premium now pitches custom
  domains + Tor only.
- setup.sh / docs/mail.md / deploy/mailu: address-domain vs server-host split,
  Mailu API token, MX for the address domain, local-relay SMTP for verify codes.

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

* chore(mailu): pin Docker network subnet to match SUBNET; ignore runtime state

The base compose declares no network, so Docker assigns the default bridge an
arbitrary subnet that won't match mailu.env SUBNET — breaking Mailu's internal
service auth/relay. Add a docker-compose.override.yml.example that pins the
default network to 192.168.203.0/24, and gitignore the live override + Mailu
runtime state (mailu.env, certs/, data/).

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

* feat(mail): plaintext loopback IMAP so the gateway bypasses Mailu's front

Mailu's front (nginx mail proxy) pre-authenticates against Mailu's user DB before
proxying to Dovecot, which rejects the Dovecot master-user login <addr>*gateway.
The gateway must reach Dovecot directly. The imap container has no TLS cert (only
the front does), so the bypass is plaintext over loopback — the master password
never leaves the host.

- mailbox: IMAPConfig.Plaintext dials with DialInsecure (loopback only).
- main.go: mailClientFor sets Plaintext from AGENTBBS_MAIL_IMAP_PLAINTEXT.
- override.example: add the unbound resolver (admin needs DNSSEC), webmail image
  fix (2024.06 uses mailu/webmail), and publish Dovecot 143 on 127.0.0.1:14143.
- docs/mail.md: document the front-bypass, the dovecot.conf master passdb (Mailu
  includes that exact filename), and the 644 master-users perms (640 = temp_fail).

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

* deploy(mailu): wire gateway IMAP to the loopback Dovecot path in setup.sh

setup.sh §9e set AGENTBBS_MAIL_IMAP_ADDR to the front's :993, which the front's
auth proxy rejects for the master-user login (and would clobber the working
loopback wiring on every self-update). Point it at 127.0.0.1:14143 +
AGENTBBS_MAIL_IMAP_PLAINTEXT=1 instead, matching the override + docs.

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

* feat(mail): give free members a webmail password at join@

The gateway opens mailboxes via the Dovecot master user (no member password),
but webmail (Roundcube) needs the member to have a password. join@ now sets a
fresh, readable webmail password via the Mailu API and shows it with the webmail
URL + login, so free members can use webmail at mail.profullstack.com.

- mailu: SetPassword (PATCH /user/<email> raw_password) + test.
- main.go: setWebmailPassword + readablePassword; join@ displays url/login/password.

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

deploy/mailu/docker-compose.override.yml.example

@@ -0,0 +1,36 @@
+# docker-compose.override.yml — copy to docker-compose.override.yml (gitignored).
+# Compose loads this file automatically. It carries three fixes the trimmed base
+# compose needs; see docs/mail.md for the full rationale.
+networks:
+  default:
+    driver: bridge
+    ipam:
+      config:
+        # Mailu trusts SUBNET (mailu.env) as its internal network for
+        # service-to-service auth/relay; the real network MUST match it.
+        - subnet: 192.168.203.0/24
+services:
+  # Mailu requires a DNSSEC-validating resolver or admin won't start.
+  resolver:
+    image: ghcr.io/mailu/unbound:2024.06
+    env_file: mailu.env
+    restart: always
+    networks:
+      default:
+        ipv4_address: 192.168.203.254
+  front:    { dns: [192.168.203.254], depends_on: [resolver] }
+  admin:    { dns: [192.168.203.254], depends_on: [resolver] }
+  imap:
+    dns: [192.168.203.254]
+    depends_on: [resolver]
+    # Publish Dovecot directly on loopback so the agentbbs gateway can use the
+    # master-user login (the front's nginx auth proxy rejects "<addr>*master").
+    # Plaintext is fine: the connection never leaves the host.
+    ports: ["127.0.0.1:14143:143"]
+  smtp:     { dns: [192.168.203.254], depends_on: [resolver] }
+  antispam: { dns: [192.168.203.254], depends_on: [resolver] }
+  # In Mailu 2024.06 the webmail image is "webmail" (not "roundcube:2024.06").
+  webmail:
+    image: ghcr.io/mailu/webmail:2024.06
+    dns: [192.168.203.254]
+    depends_on: [resolver]