feat(broadcasting): add ActionCable adapter + fix model broadcast anti-pattern

- Add Mozo::Broadcaster::ActionCable as drop-in Faye replacement
- Fix model_broadcast.rb: delegate to Mozo directly instead of
  ApplicationController.new (memory-unsafe anti-pattern)
- Add Broadcastable concern for clean model-side broadcasting
- ActionCable config: async adapter, cable.yml, WebSocket endpoint
- MozoChannel with per-entity authorization (user/supplier/employee)
- Connection auth via auth_token (matches existing auth pattern)
- Mount /cable WebSocket in routes
- Add broadcasting-migration.md with Faye→ActionCable guide

config/routes.rb

@@ -1,5 +1,9 @@
 ALLOWED_LOCALES = /nl|de|fr|en|es/
 Mozo::Application.routes.draw do
+  # ActionCable WebSocket endpoint (replaces Faye at events.mozo.bar/faye)
+  # Clients connect via: wss://mozo.bar/cable?auth_token=TOKEN
+  mount ActionCable.server => '/cable'
+
   match '/.well-known/*rest', to: 'errors#not_found', via: :all
   match '/system/*rest',      to: 'errors#not_found', via: :all
   devise_for :users, controllers: {