Return character limit in API and initial state.

app/serializers/rest/instance_serializer.rb

@@ -4,7 +4,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :uri, :title, :description, :email,
-             :version, :urls, :stats, :thumbnail
+             :version, :urls, :stats, :thumbnail, :max_chars
 
   def uri
     Rails.configuration.x.local_domain
@@ -30,6 +30,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     full_asset_url(instance_presenter.thumbnail.file.url) if instance_presenter.thumbnail
   end
 
+  def max_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
   def stats
     {
       user_count: instance_presenter.user_count,