add create signup code

Femto.Modules.Auth/Models/SignupCode.cs

@@ -0,0 +1,41 @@
+using Femto.Common.Domain;
+
+namespace Femto.Modules.Auth.Models;
+
+public class SignupCode
+{
+    private static readonly TimeSpan ExpiryTime = TimeSpan.FromDays(14);
+    public string Code { get; private set; }
+    
+    /// <summary>
+    /// The email of the intended recipient
+    /// </summary>
+    public string RecipientEmail { get; private set; }
+    
+    /// <summary>
+    /// The name of the intended recipient
+    /// </summary>
+    public string RecipientName { get; private set; }
+    public DateTimeOffset CreatedAt { get; private set; }
+    public DateTimeOffset? ExpiresAt { get; private set; }
+    public Guid? RedeemingUserId { get; private set; }
+
+    private SignupCode() { }
+    
+    public SignupCode(string recipientEmail, string recipientName, string code)
+    {
+        this.Code = code;
+        this.RecipientEmail = recipientEmail;
+        this.RecipientName = recipientName;
+        this.CreatedAt = DateTimeOffset.UtcNow;
+        this.ExpiresAt = this.CreatedAt + ExpiryTime;
+    }
+
+    public void Redeem(Guid userGuid)
+    {
+        if (this.RedeemingUserId is not null) 
+            throw new DomainError("invalid signup code");
+        
+        this.RedeemingUserId = userGuid;
+    }
+}