feat(backend): add add email address endpoint

Foxnouns.Backend/Services/AuthService.cs

@@ -129,6 +129,30 @@ public class AuthService(IClock clock, DatabaseContext db, ISnowflakeGenerator s
         return (user, EmailAuthenticationResult.AuthSuccessful);
     }
 
+    /// <summary>
+    /// Validates a user's password outside an authentication context, for when a password is required for changing
+    /// a setting, such as adding a new email address or changing passwords.
+    /// </summary>
+    public async Task<bool> ValidatePasswordAsync(
+        User user,
+        string password,
+        CancellationToken ct = default
+    )
+    {
+        if (user.Password == null)
+        {
+            throw new FoxnounsError("Password for user supplied to ValidatePasswordAsync was null");
+        }
+
+        var pwResult = await Task.Run(
+            () => _passwordHasher.VerifyHashedPassword(user, user.Password!, password),
+            ct
+        );
+        return pwResult
+            is PasswordVerificationResult.SuccessRehashNeeded
+                or PasswordVerificationResult.Success;
+    }
+
     public enum EmailAuthenticationResult
     {
         AuthSuccessful,

Foxnouns.Backend/Services/MailService.cs

@@ -33,4 +33,31 @@ public class MailService(ILogger logger, IMailer mailer, IQueue queue, Config co
             }
         });
     }
+
+    public void QueueAddEmailAddressEmail(string to, string code, string username)
+    {
+        _logger.Debug("Sending add email address email to {ToEmail}", to);
+        queue.QueueAsyncTask(async () =>
+        {
+            try
+            {
+                await mailer.SendAsync(
+                    new AddEmailMailable(
+                        config,
+                        new AddEmailMailableView
+                        {
+                            BaseUrl = config.BaseUrl,
+                            To = to,
+                            Code = code,
+                            Username = username,
+                        }
+                    )
+                );
+            }
+            catch (Exception exc)
+            {
+                _logger.Error(exc, "Sending add email address email");
+            }
+        });
+    }
 }