fix: fail over mail server addresses

[YunQue0912]

Summary

Fixes #318.

Mail server hostnames can resolve to several valid addresses, but MailFlow previously pinned every connection to the first one. An unreachable first address therefore blocked IMAP and SMTP even when another validated address was healthy. SMTP's default connection timeout could also outlive the frontend proxy timeout.

Changes

• Preserve every validated A/AAAA address while keeping DNS rebinding protection: connection code receives only the addresses validated up front.
• Let Node try the remaining validated IMAP addresses while retaining the original hostname for TLS SNI and certificate verification.
• Add one SMTP transport wrapper for user mail, system mail, invite/password-reset mail, and SMTP verification.
• Limit each SMTP connection and greeting stage to 10 seconds and the failover sequence to a 45-second budget.
• Retry SMTP only for Nodemailer CONN failures, before authentication or envelope/data commands, so failover cannot duplicate delivered messages.

Testing

• Backend lint: npm run lint
• Backend syntax: node --check src/index.js
• Backend tests: 710 passed across 35 files
• Backend production audit: 0 vulnerabilities
• Frontend lint: npm run lint
• Frontend build: npm run build
• Frontend tests: 1401 passed across 48 suites
• Frontend production audit at audit-level=high: passed (two existing moderate React Router advisories have no available fix)
• Added a local socket test proving same-family IPv4 fallback reaches the second candidate without another DNS lookup
• git diff --check

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