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

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

@@ -0,0 +1,12 @@
+using Features.Auth.Dtos;
+using MediatR;
+
+namespace Features.Auth.Commands.UpdateUsername;
+
+/// <summary>Changes the username of the given user account.</summary>
+/// <param name="UserAccountId">
+///     The authenticated caller's own ID, extracted from the access token -- never bind this from
+///     client-supplied input.
+/// </param>
+public record UpdateUsernameCommand(Guid UserAccountId, string NewUsername)
+    : IRequest<UpdateUsernamePayload>;