Add complete email service infrastructure with Mock and SMTP implementations.
Changes:
- Created EmailMessage domain model for email data
- Added IEmailService interface for email sending
- Implemented MockEmailService for development/testing (logs emails)
- Implemented SmtpEmailService for production SMTP sending
- Added IEmailTemplateService interface for email templates
- Implemented EmailTemplateService with HTML templates for verification, password reset, and invitation emails
- Registered email services in DependencyInjection with provider selection
- Added email configuration to appsettings.Development.json (Mock provider by default)
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Co-Authored-By: Claude <noreply@anthropic.com>
