feat: use a FixedWindowRateLimiter keyed by IP to rate limit emails

we don't talk about the sent_emails table :)

Foxnouns.Backend/Database/Models/SentEmail.cs

@@ -1,13 +0,0 @@
-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; }
-}