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.Application/Services/IEmailService.cs

@@ -0,0 +1,17 @@
+using ColaFlow.Modules.Identity.Domain.Services;
+
+namespace ColaFlow.Modules.Identity.Application.Services;
+
+/// <summary>
+/// Service for sending emails
+/// </summary>
+public interface IEmailService
+{
+    /// <summary>
+    /// Sends an email message
+    /// </summary>
+    /// <param name="message">The email message to send</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>True if email was sent successfully, false otherwise</returns>
+    Task<bool> SendEmailAsync(EmailMessage message, CancellationToken cancellationToken = default);
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Application/Services/IEmailTemplateService.cs

@@ -0,0 +1,37 @@
+namespace ColaFlow.Modules.Identity.Application.Services;
+
+/// <summary>
+/// Service for rendering email templates
+/// </summary>
+public interface IEmailTemplateService
+{
+    /// <summary>
+    /// Renders a verification email template
+    /// </summary>
+    /// <param name="recipientName">Name of the recipient</param>
+    /// <param name="verificationUrl">URL for email verification</param>
+    /// <returns>HTML email body</returns>
+    string RenderVerificationEmail(string recipientName, string verificationUrl);
+
+    /// <summary>
+    /// Renders a password reset email template
+    /// </summary>
+    /// <param name="recipientName">Name of the recipient</param>
+    /// <param name="resetUrl">URL for password reset</param>
+    /// <returns>HTML email body</returns>
+    string RenderPasswordResetEmail(string recipientName, string resetUrl);
+
+    /// <summary>
+    /// Renders a tenant invitation email template
+    /// </summary>
+    /// <param name="recipientName">Name of the recipient</param>
+    /// <param name="tenantName">Name of the tenant</param>
+    /// <param name="inviterName">Name of the person who sent the invitation</param>
+    /// <param name="invitationUrl">URL for accepting the invitation</param>
+    /// <returns>HTML email body</returns>
+    string RenderInvitationEmail(
+        string recipientName,
+        string tenantName,
+        string inviterName,
+        string invitationUrl);
+}