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

app/models/concerns/broadcastable.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# Include this in any model that needs to broadcast events to users/suppliers.
+#
+# Replaces the old model_broadcast.rb initializer which monkey-patched
+# SimplyStored::Couch and created ApplicationController.new per broadcast
+# (memory-unsafe, no request context, to be removed once all callers migrate).
+#
+# Usage:
+#   class List < ApplicationRecord
+#     include Broadcastable
+#
+#     def close!
+#       broadcast_user(user.id, 'list_closed', { id: id })
+#       broadcast_supplier(supplier_id, 'list_closed', { id: id })
+#     end
+#   end
+#
+module Broadcastable
+  extend ActiveSupport::Concern
+
+  def broadcast_supplier(sid, event, data = {})
+    Mozo.broadcast_supplier(sid, event, data)
+  end
+
+  def broadcast_user(uid, event, data = {})
+    Mozo.broadcast_user(uid, event, data)
+  end
+end