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/API/API.Specs/Mocks/MockEmailService.cs

@@ -0,0 +1,54 @@
+using Infrastructure.Email;
+
+namespace API.Specs.Mocks;
+
+/// <summary>
+/// Mock email service for testing that doesn't actually send emails.
+/// Tracks sent emails for verification in tests if needed.
+/// </summary>
+public class MockEmailProvider : IEmailProvider
+{
+   public List<SentEmail> SentEmails { get; } = new();
+
+   public Task SendAsync(string to, string subject, string body, bool isHtml = true)
+   {
+      SentEmails.Add(new SentEmail
+      {
+         To = [to],
+         Subject = subject,
+         Body = body,
+         IsHtml = isHtml,
+         SentAt = DateTime.UtcNow
+      });
+
+      return Task.CompletedTask;
+   }
+
+   public Task SendAsync(IEnumerable<string> to, string subject, string body, bool isHtml = true)
+   {
+      SentEmails.Add(new SentEmail
+      {
+         To = to.ToList(),
+         Subject = subject,
+         Body = body,
+         IsHtml = isHtml,
+         SentAt = DateTime.UtcNow
+      });
+
+      return Task.CompletedTask;
+   }
+
+   public void Clear()
+   {
+      SentEmails.Clear();
+   }
+
+   public class SentEmail
+   {
+      public List<string> To { get; init; } = new();
+      public string Subject { get; init; } = string.Empty;
+      public string Body { get; init; } = string.Empty;
+      public bool IsHtml { get; init; }
+      public DateTime SentAt { get; init; }
+   }
+}