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

docs/broadcasting-migration.md

@@ -0,0 +1,89 @@
+# Broadcasting: Faye → ActionCable Migration Guide
+
+## Current state
+
+```
+Model (SimplyStored::Couch)
+  → ApplicationController.new.broadcast_user   # ⚠️ anti-pattern
+    → Mozo.broadcast_user
+      → Mozo::Broadcaster::Faye.new.broadcast  # HTTP POST to Faye
+        → Faye server (Thin, port 9296)
+          → WebSocket → browser clients
+```
+
+## Target state
+
+```
+Model (Broadcastable concern)
+  → Mozo.broadcast_user
+    → Mozo::Broadcaster::ActionCable.new.broadcast  # in-process async
+      → ActionCable (Rails built-in)
+        → WebSocket → browser clients
+```
+
+## What this branch adds
+
+| File | Purpose |
+|------|---------|
+| `lib/mozo/broadcaster/action_cable.rb` | Drop-in ActionCable broadcaster adapter |
+| `config/cable.yml` | ActionCable configuration (async for single-server) |
+| `app/channels/application_cable/connection.rb` | WebSocket auth via auth_token |
+| `app/channels/mozo_channel.rb` | Channel authorization for user/supplier/employee |
+| `app/models/concerns/broadcastable.rb` | Clean module for models (replaces old monkey-patch) |
+| `config/routes.rb` | Mounts `/cable` WebSocket endpoint |
+| `config/initializers/model_broadcast.rb` | Fixed: delegates to Mozo directly (no more `ApplicationController.new`) |
+
+## How to switch
+
+### 1. Server (one-line change)
+
+In `config/initializers/mozo_settings.rb`, change:
+
+```ruby
+# Mozo.broadcaster = Mozo::Broadcaster::Faye.new  # old
+Mozo.broadcaster = Mozo::Broadcaster::ActionCable.new  # new
+```
+
+### 2. Client (mozo-user / mozo-supplier)
+
+**Old Faye client (conceptual):**
+```js
+var client = new Faye.Client('https://events.mozo.bar/faye');
+client.subscribe('/user/123', function(msg) { ... });
+```
+
+**New ActionCable client:**
+```js
+// Using @rails/actioncable npm package
+import { createConsumer } from "@rails/actioncable";
+
+const consumer = createConsumer(
+  `wss://mozo.bar/cable?auth_token=${authToken}`
+);
+
+consumer.subscriptions.create(
+  { channel: "MozoChannel", id: "user_123" },
+  {
+    received(data) {
+      // data = { event: "list_closed", data: { id: 42 } }
+      handleEvent(data.event, data.data);
+    }
+  }
+);
+```
+
+### 3. Remove Faye
+
+Once stable:
+- Remove `gem 'faye'` from Gemfile
+- Remove `faye/` directory
+- Remove nginx `events.mozo.bar` vhost
+- Stop the Faye Thin process
+
+## Benefits
+
+- **No extra process** — ActionCable runs inside Puma
+- **Async** — `broadcast` is non-blocking
+- **Simpler deploys** — one less service to manage
+- **WebSocket native** — no long-polling fallback complexity
+- **Rails auth** — cookies/sessions work automatically