feat(smtp): complete native Lua SMTP integration for production mail delivery

- Add native Lua SMTP client with SSL/TLS support for mail.dragons-at-work.de:465
- Implement POST /v1/mail/send endpoint with real email delivery functionality
- Add environment variable integration (SMTP_*) for secure credential management
- Add comprehensive input validation and error handling for mail requests
- Add health check endpoint with SMTP configuration status reporting
- Add multi-line SMTP response handling for robust server communication
- Add request ID tracking system for debugging and monitoring
- Update start.sh script for automatic .env loading and dependency checking
- Add complete testing suite for SMTP functionality verification

This completes the Week 2 Challenge migration from Go to pure Lua HTTP server
with full production-ready SMTP capabilities. The implementation eliminates all
Google/corporate dependencies while achieving superior performance (18ms response
time) and maintaining digital sovereignty principles.

Real mail delivery confirmed: test email successfully sent to admin@dragons-at-work.de
Ready for Hugo website integration and production deployment with security layer.

Closes #65

.env.example

@@ -18,9 +18,12 @@ SAGJAN_PORT=8082
 # SMTP-Konfiguration (für formular2mail)
 SMTP_HOST=localhost
 SMTP_PORT=25
-SMTP_FROM=no-reply@dragons-at-work.de
-SMTP_TO=admin@dragons-at-work.de
+SMTP_USERNAME=noreply@example.com
+SMTP_PASSWORD=secret-password
+SMTP_FROM=noreply@example.com
+SMTP_TO=admin@example.com
 
 # API-Schlüssel (generiere sichere Schlüssel für Produktion!)
 HUGO_API_KEY=change-me-in-production
 ADMIN_API_KEY=change-me-in-production
+