feat: rate limit emails to two per address per hour

Foxnouns.Backend/Database/Models/SentEmail.cs

@@ -0,0 +1,13 @@
+using System.ComponentModel.DataAnnotations.Schema;
+using NodaTime;
+
+namespace Foxnouns.Backend.Database.Models;
+
+public class SentEmail
+{
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    public int Id { get; init; }
+
+    public required string Email { get; init; }
+    public required Instant SentAt { get; init; }
+}