Gate live-content changes on restricted surfaces

publish_draft!, trash!, destroy_from_trash!, attach_asset! and
Asset#destroy_witnessed! now refuse unless the acting user holds redaktion,
and only when the subject is on a restricted surface: the front page, the
updates tree that feeds ~100k subscribers, or disclosure. Drafting,
autosaving, tagging and creating stay free everywhere for everyone.

Enforcement is in the models rather than the controllers, since attach_asset!
and the rest are reachable from rake tasks and internal paths. It follows the
errors.add-plus-bare-raise pattern the rest of Node already uses, so every
existing RecordInvalid rescue reports it with a localised message; only
assets_controller#destroy needed a rescue added.

A nil user is treated as a system context and bypasses the gate. The default
nil on three of those verbs is what makes that reachable, and removing those
defaults once every call site passes a user is the next tightening.

1 files changed, 5 insertions, 0 deletions

diff --git a/app/models/user.rb b/app/models/user.rb
index 1728521a..e8c3b9bb 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -105,6 +105,11 @@ class User < ApplicationRecord
     roles.map { |r| I18n.t("users.roles.#{r}", :default => r) }
   end
 
+  def may_change_live?(subject)
+    return true unless subject.restricted?
+    redaktion?
+  end
+
   def deactivate!(actor:)
     return false if alumni?
     transaction do