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/ColaFlow.API/appsettings.Development.json

@@ -10,6 +10,18 @@
     "PMDatabase": "Host=localhost;Port=5432;Database=colaflow_pm;Username=colaflow;Password=colaflow_dev_password",
     "DefaultConnection": "Host=localhost;Port=5432;Database=colaflow_identity;Username=colaflow;Password=colaflow_dev_password"
   },
+  "Email": {
+    "Provider": "Mock",
+    "From": "noreply@colaflow.local",
+    "FromName": "ColaFlow",
+    "Smtp": {
+      "Host": "smtp.example.com",
+      "Port": "587",
+      "Username": "",
+      "Password": "",
+      "EnableSsl": "true"
+    }
+  },
   "MediatR": {
     "LicenseKey": "eyJhbGciOiJSUzI1NiIsImtpZCI6Ikx1Y2t5UGVubnlTb2Z0d2FyZUxpY2Vuc2VLZXkvYmJiMTNhY2I1OTkwNGQ4OWI0Y2IxYzg1ZjA4OGNjZjkiLCJ0eXAiOiJKV1QifQ.eyJpc3MiOiJodHRwczovL2x1Y2t5cGVubnlzb2Z0d2FyZS5jb20iLCJhdWQiOiJMdWNreVBlbm55U29mdHdhcmUiLCJleHAiOiIxNzkzNTc3NjAwIiwiaWF0IjoiMTc2MjEyNTU2MiIsImFjY291bnRfaWQiOiIwMTlhNDZkZGZiZjk3YTk4Yjg1ZTVmOTllNWRhZjIwNyIsImN1c3RvbWVyX2lkIjoiY3RtXzAxazkzZHdnOG0weDByanp3Mm5rM2dxeDExIiwic3ViX2lkIjoiLSIsImVkaXRpb24iOiIwIiwidHlwZSI6IjIifQ.V45vUlze27pQG3Vs9dvagyUTSp-a74ymB6I0TIGD_NwFt1mMMPsuVXOKH1qK7A7V5qDQBvYyryzJy8xRE1rRKq2MJKgyfYjvzuGkpBbKbM6JRQPYknb5tjF-Rf3LAeWp73FiqbPZOPt5saCsoKqUHej-4zcKg5GA4y-PpGaGAONKyqwK9G2rvc1BUHfEnHKRMr0pprA5W1Yx-Lry85KOckUsI043HGOdfbubnGdAZs74FKvrV2qVir6K6VsZjWwX8IFnl1CzxjICa5MxyHOAVpXRnRtMt6fpsA1fMstFuRjq_2sbqGfsTv6LyCzLPnXdmU5DnWZHUcjy0xlAT_f0aw"
   },

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);
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Domain/Services/EmailMessage.cs

@@ -0,0 +1,13 @@
+namespace ColaFlow.Modules.Identity.Domain.Services;
+
+/// <summary>
+/// Represents an email message to be sent
+/// </summary>
+public sealed record EmailMessage(
+    string To,
+    string Subject,
+    string HtmlBody,
+    string? PlainTextBody = null,
+    string? FromEmail = null,
+    string? FromName = null
+);

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;
     }
 }

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Infrastructure/Services/EmailTemplateService.cs

@@ -0,0 +1,150 @@
+using ColaFlow.Modules.Identity.Application.Services;
+
+namespace ColaFlow.Modules.Identity.Infrastructure.Services;
+
+/// <summary>
+/// Service for rendering HTML email templates
+/// </summary>
+public sealed class EmailTemplateService : IEmailTemplateService
+{
+    public string RenderVerificationEmail(string recipientName, string verificationUrl)
+    {
+        return $@"
+<!DOCTYPE html>
+<html>
+<head>
+    <meta charset=""utf-8"">
+    <meta name=""viewport"" content=""width=device-width, initial-scale=1.0"">
+    <title>Verify Your Email</title>
+    <style>
+        body {{ font-family: Arial, sans-serif; line-height: 1.6; color: #333; }}
+        .container {{ max-width: 600px; margin: 0 auto; padding: 20px; }}
+        .header {{ background-color: #4F46E5; color: white; padding: 20px; text-align: center; border-radius: 8px 8px 0 0; }}
+        .content {{ background-color: #f9fafb; padding: 30px; border-radius: 0 0 8px 8px; }}
+        .button {{ display: inline-block; background-color: #4F46E5; color: white; padding: 12px 30px; text-decoration: none; border-radius: 6px; margin: 20px 0; }}
+        .footer {{ text-align: center; margin-top: 20px; font-size: 12px; color: #666; }}
+    </style>
+</head>
+<body>
+    <div class=""container"">
+        <div class=""header"">
+            <h1>ColaFlow</h1>
+        </div>
+        <div class=""content"">
+            <h2>Welcome to ColaFlow, {recipientName}!</h2>
+            <p>Thank you for registering. Please verify your email address by clicking the button below:</p>
+            <div style=""text-align: center;"">
+                <a href=""{verificationUrl}"" class=""button"">Verify Email Address</a>
+            </div>
+            <p>Or copy and paste this link into your browser:</p>
+            <p style=""word-break: break-all; color: #4F46E5;"">{verificationUrl}</p>
+            <p>This link will expire in 24 hours.</p>
+            <p>If you didn't create an account, you can safely ignore this email.</p>
+        </div>
+        <div class=""footer"">
+            <p>&copy; 2025 ColaFlow. All rights reserved.</p>
+        </div>
+    </div>
+</body>
+</html>";
+    }
+
+    public string RenderPasswordResetEmail(string recipientName, string resetUrl)
+    {
+        return $@"
+<!DOCTYPE html>
+<html>
+<head>
+    <meta charset=""utf-8"">
+    <meta name=""viewport"" content=""width=device-width, initial-scale=1.0"">
+    <title>Reset Your Password</title>
+    <style>
+        body {{ font-family: Arial, sans-serif; line-height: 1.6; color: #333; }}
+        .container {{ max-width: 600px; margin: 0 auto; padding: 20px; }}
+        .header {{ background-color: #DC2626; color: white; padding: 20px; text-align: center; border-radius: 8px 8px 0 0; }}
+        .content {{ background-color: #f9fafb; padding: 30px; border-radius: 0 0 8px 8px; }}
+        .button {{ display: inline-block; background-color: #DC2626; color: white; padding: 12px 30px; text-decoration: none; border-radius: 6px; margin: 20px 0; }}
+        .footer {{ text-align: center; margin-top: 20px; font-size: 12px; color: #666; }}
+        .warning {{ background-color: #FEF3C7; border-left: 4px solid #F59E0B; padding: 12px; margin: 20px 0; }}
+    </style>
+</head>
+<body>
+    <div class=""container"">
+        <div class=""header"">
+            <h1>ColaFlow</h1>
+        </div>
+        <div class=""content"">
+            <h2>Password Reset Request</h2>
+            <p>Hi {recipientName},</p>
+            <p>We received a request to reset your password. Click the button below to create a new password:</p>
+            <div style=""text-align: center;"">
+                <a href=""{resetUrl}"" class=""button"">Reset Password</a>
+            </div>
+            <p>Or copy and paste this link into your browser:</p>
+            <p style=""word-break: break-all; color: #DC2626;"">{resetUrl}</p>
+            <div class=""warning"">
+                <strong>Important:</strong> This link will expire in 1 hour for security reasons.
+            </div>
+            <p>If you didn't request a password reset, please ignore this email or contact support if you have concerns.</p>
+        </div>
+        <div class=""footer"">
+            <p>&copy; 2025 ColaFlow. All rights reserved.</p>
+        </div>
+    </div>
+</body>
+</html>";
+    }
+
+    public string RenderInvitationEmail(
+        string recipientName,
+        string tenantName,
+        string inviterName,
+        string invitationUrl)
+    {
+        return $@"
+<!DOCTYPE html>
+<html>
+<head>
+    <meta charset=""utf-8"">
+    <meta name=""viewport"" content=""width=device-width, initial-scale=1.0"">
+    <title>You've Been Invited</title>
+    <style>
+        body {{ font-family: Arial, sans-serif; line-height: 1.6; color: #333; }}
+        .container {{ max-width: 600px; margin: 0 auto; padding: 20px; }}
+        .header {{ background-color: #10B981; color: white; padding: 20px; text-align: center; border-radius: 8px 8px 0 0; }}
+        .content {{ background-color: #f9fafb; padding: 30px; border-radius: 0 0 8px 8px; }}
+        .button {{ display: inline-block; background-color: #10B981; color: white; padding: 12px 30px; text-decoration: none; border-radius: 6px; margin: 20px 0; }}
+        .footer {{ text-align: center; margin-top: 20px; font-size: 12px; color: #666; }}
+        .info-box {{ background-color: #E0F2FE; border-left: 4px solid #0EA5E9; padding: 12px; margin: 20px 0; }}
+    </style>
+</head>
+<body>
+    <div class=""container"">
+        <div class=""header"">
+            <h1>ColaFlow</h1>
+        </div>
+        <div class=""content"">
+            <h2>You've Been Invited!</h2>
+            <p>Hi {recipientName},</p>
+            <p><strong>{inviterName}</strong> has invited you to join the <strong>{tenantName}</strong> workspace on ColaFlow.</p>
+            <div class=""info-box"">
+                <strong>Workspace:</strong> {tenantName}<br>
+                <strong>Invited by:</strong> {inviterName}
+            </div>
+            <p>Click the button below to accept the invitation and get started:</p>
+            <div style=""text-align: center;"">
+                <a href=""{invitationUrl}"" class=""button"">Accept Invitation</a>
+            </div>
+            <p>Or copy and paste this link into your browser:</p>
+            <p style=""word-break: break-all; color: #10B981;"">{invitationUrl}</p>
+            <p>This invitation will expire in 7 days.</p>
+            <p>If you don't know {inviterName} or weren't expecting this invitation, you can safely ignore this email.</p>
+        </div>
+        <div class=""footer"">
+            <p>&copy; 2025 ColaFlow. All rights reserved.</p>
+        </div>
+    </div>
+</body>
+</html>";
+    }
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Infrastructure/Services/MockEmailService.cs

@@ -0,0 +1,34 @@
+using ColaFlow.Modules.Identity.Application.Services;
+using ColaFlow.Modules.Identity.Domain.Services;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Infrastructure.Services;
+
+/// <summary>
+/// Mock email service for development/testing that logs emails instead of sending them
+/// </summary>
+public sealed class MockEmailService : IEmailService
+{
+    private readonly ILogger<MockEmailService> _logger;
+
+    public MockEmailService(ILogger<MockEmailService> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task<bool> SendEmailAsync(EmailMessage message, CancellationToken cancellationToken = default)
+    {
+        _logger.LogInformation(
+            "[MOCK EMAIL] To: {To}, Subject: {Subject}, From: {From}",
+            message.To,
+            message.Subject,
+            message.FromEmail ?? "default");
+
+        _logger.LogDebug(
+            "[MOCK EMAIL] HTML Body: {HtmlBody}",
+            message.HtmlBody);
+
+        // Simulate successful send
+        return Task.FromResult(true);
+    }
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Infrastructure/Services/SmtpEmailService.cs

@@ -0,0 +1,87 @@
+using System.Net;
+using System.Net.Mail;
+using ColaFlow.Modules.Identity.Application.Services;
+using ColaFlow.Modules.Identity.Domain.Services;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Infrastructure.Services;
+
+/// <summary>
+/// SMTP-based email service for production use
+/// </summary>
+public sealed class SmtpEmailService : IEmailService
+{
+    private readonly ILogger<SmtpEmailService> _logger;
+    private readonly IConfiguration _configuration;
+
+    public SmtpEmailService(
+        ILogger<SmtpEmailService> logger,
+        IConfiguration configuration)
+    {
+        _logger = logger;
+        _configuration = configuration;
+    }
+
+    public async Task<bool> SendEmailAsync(EmailMessage message, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var smtpHost = _configuration["Email:Smtp:Host"];
+            var smtpPort = int.Parse(_configuration["Email:Smtp:Port"] ?? "587");
+            var smtpUsername = _configuration["Email:Smtp:Username"];
+            var smtpPassword = _configuration["Email:Smtp:Password"];
+            var enableSsl = bool.Parse(_configuration["Email:Smtp:EnableSsl"] ?? "true");
+
+            var defaultFromEmail = _configuration["Email:From"] ?? "noreply@colaflow.local";
+            var defaultFromName = _configuration["Email:FromName"] ?? "ColaFlow";
+
+            using var smtpClient = new SmtpClient(smtpHost, smtpPort)
+            {
+                Credentials = new NetworkCredential(smtpUsername, smtpPassword),
+                EnableSsl = enableSsl
+            };
+
+            using var mailMessage = new MailMessage
+            {
+                From = new MailAddress(
+                    message.FromEmail ?? defaultFromEmail,
+                    message.FromName ?? defaultFromName),
+                Subject = message.Subject,
+                Body = message.HtmlBody,
+                IsBodyHtml = true
+            };
+
+            mailMessage.To.Add(message.To);
+
+            // Add plain text alternative if provided
+            if (!string.IsNullOrEmpty(message.PlainTextBody))
+            {
+                var plainView = AlternateView.CreateAlternateViewFromString(
+                    message.PlainTextBody,
+                    null,
+                    "text/plain");
+                mailMessage.AlternateViews.Add(plainView);
+            }
+
+            await smtpClient.SendMailAsync(mailMessage, cancellationToken);
+
+            _logger.LogInformation(
+                "Email sent successfully to {To} with subject: {Subject}",
+                message.To,
+                message.Subject);
+
+            return true;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(
+                ex,
+                "Failed to send email to {To} with subject: {Subject}",
+                message.To,
+                message.Subject);
+
+            return false;
+        }
+    }
+}