fix: remove STARTTLS config for IONOS port 465

Port 465 uses implicit TLS, not STARTTLS.
Mailpit auto-detects TLS for port 465.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

net/compose.yaml

@@ -240,7 +240,6 @@ services:
       MP_SMTP_RELAY_USER: ${EMAIL_SMTP_USER}
       MP_SMTP_RELAY_PASS: ${EMAIL_SMTP_PASSWORD}
       MP_SMTP_RELAY_AUTH: plain
-      MP_SMTP_RELAY_STARTTLS: "true"
     volumes:
       - mailpit_data:/data
     networks: