feat: MIME email composer with .ovpn attachment via msmtp/sendmail

Add send_email() function that composes multipart MIME messages with
template-based body and .ovpn file attachment, piped to mail binary via
subprocess.Popen. Supports Cryptgeon URL insertion and sender metadata.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

email_template.txt

@@ -0,0 +1,11 @@
+Hello,
+
+Your OpenVPN configuration has been updated for: {cn}
+
+Retrieve your one-time password here (link expires after one view):
+{url}
+
+Your configuration file is attached as '{config_name}'.
+
+Regards,
+VPN Admin