Allow Capybara test server host in HostAuthorization

Capybara starts a Puma server on 127.0.0.1:<random_port> for system
tests. Rails HostAuthorization was blocking all requests from that host
since it was not in the config.hosts allowlist, causing every page to
return a blank 403 response and all system tests to fail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/system/users_test.rb

@@ -229,7 +229,7 @@ class UsersTest < ApplicationSystemTestCase
       user = User.find_by_email!(first(:link).text)
       inject_button_to first('td:not(.link)'), "update status", user_path(user), method: :patch,
         params: {user: {status: User.statuses.keys.sample}}, data: {turbo: false}
-      execute_script("arguments[0].click()", find_button("update status"))
+      click_on "update status"
     end
     assert_title 'The change you wanted was rejected (422)'
   end