Make character limit configurable.

.env.production.sample

@@ -134,3 +134,6 @@ STREAMING_CLUSTER_NUM=1
 # If you use Docker, you may want to assign UID/GID manually.
 # UID=1000
 # GID=1000
+
+# Maximum allowed character count
+# MAX_CHARS=500

app/validators/status_length_validator.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class StatusLengthValidator < ActiveModel::Validator
-  MAX_CHARS = 500
+  MAX_CHARS = (ENV["MAX_CHARS"] || 500).to_i
 
   def validate(status)
     return unless status.local? && !status.reblog?