Replace SendGrid with SMTP2Go for email delivery

[Copilot]

This PR migrates the application from SendGrid to SMTP2Go for email delivery to address the discontinuation of SendGrid's free tier as of June 2025, which was causing email bounces.

Changes Made

Core Implementation

• New Service: Created Smtp2GoEmailService that implements the existing IEmailService interface using standard .NET SmtpClient with SSL/TLS support
• Configuration Update: Replaced AuthMessageSenderOptions properties from SendGrid API keys (SendGridUser, SendGridKey) to SMTP credentials (SmtpServer, SmtpPort, SmtpUsername, SmtpPassword)
• Service Registration: Updated production environment to use Smtp2GoEmailService instead of SendGridEmailService
• Dependency Cleanup: Removed SendGrid package reference from Infrastructure project

Configuration Templates

Updated all appsettings templates with SMTP2Go configuration:

"AuthMessageSenderOptions": {
  "SmtpServer": "mail.smtp2go.com",
  "SmtpPort": 587,
  "SmtpUsername": "[SMTP2Go username goes here]",
  "SmtpPassword": "[SMTP2Go password goes here]"
}

Testing & Documentation

• Added comprehensive unit tests for the new email service with proper error handling validation
• Updated README.md to reflect the email provider change
• Maintained full backward compatibility - all existing email functionality works unchanged

Technical Benefits

• More Reliable: Uses standard SMTP protocol instead of proprietary API
• Better Error Handling: Clear validation messages for missing configuration
• Secure: Uses TLS encryption for email transmission
• Cost Effective: SMTP2Go provides a more sustainable pricing model

Deployment Notes

To deploy this change, update your production configuration with valid SMTP2Go credentials. The development environment continues to use LocalSmtpEmailService unchanged.

Fixes #1384.

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