Add self-service account management commands (delete, update username/email/password/profile)

web/backend/Features/Features.Auth/Commands/UpdateUsername/UpdateUsernameValidator.cs

@@ -0,0 +1,20 @@
+using FluentValidation;
+
+namespace Features.Auth.Commands.UpdateUsername;
+
+/// <summary>Validates <see cref="UpdateUsernameCommand" />, mirroring the username rules in <c>RegisterUserValidator</c>.</summary>
+public class UpdateUsernameValidator : AbstractValidator<UpdateUsernameCommand>
+{
+    public UpdateUsernameValidator()
+    {
+        RuleFor(x => x.NewUsername)
+            .NotEmpty()
+            .WithMessage("Username is required")
+            .Length(3, 64)
+            .WithMessage("Username must be between 3 and 64 characters")
+            .Matches("^[a-zA-Z0-9._-]+$")
+            .WithMessage(
+                "Username can only contain letters, numbers, dots, underscores, and hyphens"
+            );
+    }
+}