Refactor authentication system to use username instead of email; implement change password functionality and logging; add NLog for logging support

Services/Auth/IAuthService.cs

@@ -4,5 +4,7 @@ public interface IAuthService
 {
     Task EnsureSeedUserAsync(CancellationToken cancellationToken);
 
-    Task<AuthUser?> ValidateCredentialsAsync(string email, string password, CancellationToken cancellationToken);
+    Task<AuthUser?> ValidateCredentialsAsync(string username, string password, CancellationToken cancellationToken);
+
+    Task<bool> ChangePasswordAsync(string userId, string newPassword, CancellationToken cancellationToken);
 }