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

lib/mozo/broadcaster/action_cable.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module Mozo
+  module Broadcaster
+    # Drop-in replacement for Mozo::Broadcaster::Faye that uses
+    # Rails' built-in ActionCable instead of an external Faye process.
+    #
+    # Benefits over Faye:
+    #   - Async by default (ActionCable.server.broadcast is non-blocking)
+    #   - No extra gem / process / port to manage
+    #   - Integrated with Rails authentication (cookies, sessions)
+    #   - WebSocket native (no long-polling fallback needed with modern browsers)
+    #
+    # Channel naming is kept compatible with the existing Faye convention:
+    #   /user/:uid    → user_<uid>
+    #   /supplier/:sid → supplier_<sid>
+    #
+    # To use:
+    #   Set Mozo.broadcaster = Mozo::Broadcaster::ActionCable.new
+    #   in config/initializers/mozo_settings.rb
+    #
+    class ActionCable
+      CHANNEL_PREFIX_REMAP = {
+        %r{^/user/(.+)$}     => 'user_\1',
+        %r{^/supplier/(.+)$} => 'supplier_\1'
+      }.freeze
+
+      def broadcast(message)
+        channel = message[:channel] || message['channel']
+        data    = message[:data]    || message['data']
+
+        remapped = remap_channel(channel)
+        return unless remapped
+
+        ::ActionCable.server.broadcast(remapped, data)
+      rescue => e
+        Rails.logger.error("[ACTION_CABLE][ERROR] #{e.message}")
+      end
+
+      private
+
+      def remap_channel(channel)
+        CHANNEL_PREFIX_REMAP.each do |pattern, replacement|
+          return channel.sub(pattern, replacement) if channel.match?(pattern)
+        end
+        Rails.logger.warn("[ACTION_CABLE] Unknown channel format: #{channel}")
+        nil
+      end
+    end
+  end
+end