Restore BRAND_NAME_KEY constant to AccountConfig (fork-specific, removed during upstream sync)

app/models/account_config.rb

@@ -60,6 +60,7 @@ class AccountConfig < ApplicationRecord
   TEMPLATE_CUSTOM_FIELDS_KEY = 'template_custom_fields'
   POLICY_LINKS_KEY = 'policy_links'
   ENABLE_MCP_KEY = 'enable_mcp'
+  BRAND_NAME_KEY = 'brand_name'
 
   EMAIL_VARIABLES = {
     SUBMITTER_INVITATION_EMAIL_KEY => %w[template.name submitter.link account.name].freeze,