chore: Improve email anti-spam compliance and update server docs
- Add proper email headers: X-Mailer, X-Priority, Precedence, List-Unsubscribe
- Improve HTML template with XHTML doctype for better compatibility
- Add compliance footer with unsubscribe link and address
- Remove accented characters from email template (spam trigger)
- Update CREDENCIAIS_SERVIDOR.md with:
- Current service versions (PHP 8.4.15, Nginx 1.29.4, etc)
- DNS anti-spam status (SPF ✅, DMARC ✅, DKIM ⚠️ pending)
- DKIM key for DNS publication
- Cron job documentation
- New API endpoints (/preferences)
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