feat(counter): add Redis counter adapter, replace DrbCounter

- Add Mozo::Counter::Redis with same get/set/incr/decr interface
- Add redis gem (~> 5.0) to Gemfile
- Update cable.yml to use Redis adapter in production (shared with counters)
- Document DrbCounter → Redis migration in broadcasting-migration.md
- Redis installed and running on vmi3300327
- Leave Faye as current broadcaster; both switches are one-line changes

DrbCounter problems solved:
  - In-memory → persistent (RDB + AOF)
  - Single-process DRb → multi-process safe Redis
  - Atomic INCR/DECR across Puma workers
  - One less custom process to manage

docs/broadcasting-migration.md

@@ -87,3 +87,63 @@ Once stable:
 - **Simpler deploys** — one less service to manage
 - **WebSocket native** — no long-polling fallback complexity
 - **Rails auth** — cookies/sessions work automatically
+
+---
+
+# Counter: DrbCounter → Redis Migration
+
+## Current state
+
+```
+Supplier::Counters (app/models/supplier/counters.rb)
+  → Mozo::Counter.incr/decr/get/set
+    → Mozo::Counter.connection (Mozo::DrbCounter.object)
+      → DRb → druby://localhost:9022
+        → InMemoryQCounter (separate Ruby process)
+          → on startup: reloads counts from CouchDB
+          → in-memory only (lost on restart)
+```
+
+## Problems
+
+1. **In-memory only** — restart the DRb process = lose all counts until CouchDB reload
+2. **Single-process** — DRb runs one Ruby process, single point of failure
+3. **Separate process** — another thing to monitor, deploy, and restart
+4. **Race conditions** — between Puma workers, increment/decrement is not atomic across the DRb boundary
+5. **Custom code** — `InMemoryQCounter` is 100 lines of hand-rolled counter logic
+
+## Target state
+
+```
+Supplier::Counters
+  → Mozo::Counter.incr/decr/get/set
+    → Mozo::Counter.connection (Mozo::Counter::Redis.new)
+      → Redis (localhost:6379)
+        → persistent, atomic, multi-process safe
+```
+
+## How to switch
+
+In `config/initializers/mozo_settings.rb`, change:
+
+```ruby
+# Mozo::Counter.connection = Mozo::DrbCounter.object  # old
+Mozo::Counter.connection = Mozo::Counter::Redis.new    # new
+```
+
+That's it. All existing `Mozo::Counter.get/set/incr/decr` calls work unchanged.
+
+## What Redis provides
+
+- **Atomic INCR/DECR** — no race conditions
+- **Persistence** — RDB snapshots + AOF, survives restarts
+- **Multi-process** — all Puma workers share the same Redis
+- **Already needed** — ActionCable uses Redis for pub/sub in production
+- **Battle-tested** — millions of deployments
+
+## Migration steps
+
+1. `apt-get install redis-server` — already done on vmi3300327
+2. `gem 'redis', '~> 5.0'` — added to Gemfile
+3. Switch `Mozo::Counter.connection` — one-line change in mozo_settings.rb
+4. Stop the DRb counter process (`drb_counter/drb_counter.rb`)