feat(backend): Implement email service infrastructure for Day 7

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)

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

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

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Infrastructure/DependencyInjection.cs

@@ -43,6 +43,18 @@ public static class DependencyInjection
         services.AddScoped<IPasswordHasher, PasswordHasher>();
         services.AddScoped<IRefreshTokenService, RefreshTokenService>();
 
+        // Email Services
+        var emailProvider = configuration["Email:Provider"] ?? "Mock";
+        if (emailProvider.Equals("Mock", StringComparison.OrdinalIgnoreCase))
+        {
+            services.AddSingleton<IEmailService, MockEmailService>();
+        }
+        else
+        {
+            services.AddScoped<IEmailService, SmtpEmailService>();
+        }
+        services.AddScoped<IEmailTemplateService, EmailTemplateService>();
+
         return services;
     }
 }