2.0.18: Harden mail delivery + environment-aware queue runner

SMTP transport: map smtp_security to Symfony scheme (ssl->smtps/465, tls->smtp/587) via pure SmtpTransport; fix empty from (?: not ??); guard multi-store SMTP on smtp_host; set scheme/port explicitly.

Reliability: queue path rethrows (retry/failed_jobs) with tries/timeout/backoff; sync path swallows+logs; central recipient guard; gp247_mail_send returns bool.

Secrets/UX: smtp_password rendered as password input; environment-aware reminder panel (MailQueueAdvisor) with copy-paste cron.

Queue runner: auto-register scheduled queue:work (guard != sync + GP247_SCHEDULE_QUEUE_WORK flag) so one schedule:run cron suffices on shared hosting.

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