Move hcaptcha service config to `config_for` yml (#30662)

app/controllers/concerns/auth/captcha_concern.rb

@@ -10,7 +10,7 @@ module Auth::CaptchaConcern
   end
 
   def captcha_available?
-    ENV['HCAPTCHA_SECRET_KEY'].present? && ENV['HCAPTCHA_SITE_KEY'].present?
+    Rails.configuration.x.captcha.secret_key.present? && Rails.configuration.x.captcha.site_key.present?
   end
 
   def captcha_enabled?

app/helpers/admin/settings_helper.rb

@@ -2,7 +2,7 @@
 
 module Admin::SettingsHelper
   def captcha_available?
-    ENV['HCAPTCHA_SECRET_KEY'].present? && ENV['HCAPTCHA_SITE_KEY'].present?
+    Rails.configuration.x.captcha.secret_key.present? && Rails.configuration.x.captcha.site_key.present?
   end
 
   def login_activity_title(activity)

config/application.rb

@@ -109,6 +109,7 @@ module Mastodon
       end
     end
 
+    config.x.captcha = config_for(:captcha)
     config.x.translation = config_for(:translation)
 
     config.to_prepare do

config/captcha.yml

@@ -0,0 +1,3 @@
+shared:
+  secret_key: <%= ENV.fetch('HCAPTCHA_SECRET_KEY', nil) %>
+  site_key: <%= ENV.fetch('HCAPTCHA_SITE_KEY', nil) %>