Add user registration emails + email infrastructure (#150)

* Add email functionality

* Add email template project and rendering service

* Update email template dir structure

* Add email header and footer components for user registration template

* update example env

* Refactor email templates namespace and components

* Format email dir

src/Core/Infrastructure/Infrastructure.Email/SmtpEmailProvider.cs

@@ -0,0 +1,126 @@
+using MailKit.Net.Smtp;
+using MailKit.Security;
+using MimeKit;
+
+namespace Infrastructure.Email;
+
+/// <summary>
+/// SMTP email service implementation using MailKit.
+/// Configured via environment variables.
+/// </summary>
+public class SmtpEmailProvider : IEmailProvider
+{
+    private readonly string _host;
+    private readonly int _port;
+    private readonly string? _username;
+    private readonly string? _password;
+    private readonly bool _useSsl;
+    private readonly string _fromEmail;
+    private readonly string _fromName;
+
+    public SmtpEmailProvider()
+    {
+        _host =
+            Environment.GetEnvironmentVariable("SMTP_HOST")
+            ?? throw new InvalidOperationException(
+                "SMTP_HOST environment variable is not set"
+            );
+
+        var portString =
+            Environment.GetEnvironmentVariable("SMTP_PORT") ?? "587";
+        if (!int.TryParse(portString, out _port))
+        {
+            throw new InvalidOperationException(
+                $"SMTP_PORT '{portString}' is not a valid integer"
+            );
+        }
+
+        _username = Environment.GetEnvironmentVariable("SMTP_USERNAME");
+        _password = Environment.GetEnvironmentVariable("SMTP_PASSWORD");
+
+        var useSslString =
+            Environment.GetEnvironmentVariable("SMTP_USE_SSL") ?? "true";
+        _useSsl = bool.Parse(useSslString);
+
+        _fromEmail =
+            Environment.GetEnvironmentVariable("SMTP_FROM_EMAIL")
+            ?? throw new InvalidOperationException(
+                "SMTP_FROM_EMAIL environment variable is not set"
+            );
+
+        _fromName =
+            Environment.GetEnvironmentVariable("SMTP_FROM_NAME")
+            ?? "The Biergarten";
+    }
+
+    public async Task SendAsync(
+        string to,
+        string subject,
+        string body,
+        bool isHtml = true
+    )
+    {
+        await SendAsync([to], subject, body, isHtml);
+    }
+
+    public async Task SendAsync(
+        IEnumerable<string> to,
+        string subject,
+        string body,
+        bool isHtml = true
+    )
+    {
+        var message = new MimeMessage();
+        message.From.Add(new MailboxAddress(_fromName, _fromEmail));
+
+        foreach (var recipient in to)
+        {
+            message.To.Add(MailboxAddress.Parse(recipient));
+        }
+
+        message.Subject = subject;
+
+        var bodyBuilder = new BodyBuilder();
+        if (isHtml)
+        {
+            bodyBuilder.HtmlBody = body;
+        }
+        else
+        {
+            bodyBuilder.TextBody = body;
+        }
+
+        message.Body = bodyBuilder.ToMessageBody();
+
+        using var client = new SmtpClient();
+
+        try
+        {
+            // Determine the SecureSocketOptions based on SSL setting
+            var secureSocketOptions = _useSsl
+                ? SecureSocketOptions.StartTls
+                : SecureSocketOptions.None;
+
+            await client.ConnectAsync(_host, _port, secureSocketOptions);
+
+            // Authenticate if credentials are provided
+            if (
+                !string.IsNullOrEmpty(_username)
+                && !string.IsNullOrEmpty(_password)
+            )
+            {
+                await client.AuthenticateAsync(_username, _password);
+            }
+
+            await client.SendAsync(message);
+            await client.DisconnectAsync(true);
+        }
+        catch (Exception ex)
+        {
+            throw new InvalidOperationException(
+                $"Failed to send email: {ex.Message}",
+                ex
+            );
+        }
+    }
+}