Add support for invite codes in the registration API (#27805)

app/controllers/api/v1/invites_controller.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class Api::V1::InvitesController < Api::BaseController
+  include RegistrationHelper
+
+  skip_before_action :require_authenticated_user!
+  skip_around_action :set_locale
+
+  before_action :set_invite
+  before_action :check_enabled_registrations!
+
+  # Override `current_user` to avoid reading session cookies
+  def current_user; end
+
+  def show
+    render json: { invite_code: params[:invite_code], instance_api_url: api_v2_instance_url }, status: 200
+  end
+
+  private
+
+  def set_invite
+    @invite = Invite.find_by!(code: params[:invite_code])
+  end
+
+  def check_enabled_registrations!
+    return render json: { error: I18n.t('invites.invalid') }, status: 401 unless @invite.valid_for_use?
+
+    raise Mastodon::NotPermittedError unless allowed_registration?(request.remote_ip, @invite)
+  end
+end