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/cable.yml

@@ -0,0 +1,18 @@
+# ActionCable configuration for real-time broadcasting.
+#
+# Development/Test: async adapter (in-process, no external dependency).
+# Production: async is fine for single-server deployments.
+#   Switch to Redis (`redis://...`) if scaling to multiple Puma workers
+#   where broadcasts need to reach clients connected to different workers.
+#
+development:
+  adapter: async
+
+test:
+  adapter: test
+
+production:
+  adapter: async
+  # adapter: redis
+  # url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
+  # channel_prefix: mozo_backend_production