cluster0: Gmail mail relay (msmtp + PHP SMTP), lab cron

Mirror alpine-be msmtp for CLI; compose uses Gmail SMTP for php-fpm.
Safe secrets.lab.env parser; deploy-mailrelay, install-lab-cron, verify-mail-lab.

Co-authored-by: Cursor <cursoragent@cursor.com>

sim/cluster0/mail/msmtprc.template

@@ -0,0 +1,16 @@
+# Android Cast — outbound relay (Gmail app password via msmtp.secret)
+# Installed by scripts/deploy-mailrelay.sh on cluster primary (cast01).
+defaults
+auth           on
+tls            on
+tls_trust_file /etc/ssl/certs/ca-certificates.crt
+logfile        /var/log/msmtp.log
+
+account        gmail
+host           smtp.gmail.com
+port           587
+from           __MSMTP_FROM__
+user           __MSMTP_USER__
+passwordeval   cat /etc/androidcast/msmtp.secret
+
+account default : gmail