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

config/cable.yml

@@ -1,9 +1,9 @@
 # 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:   async adapter (in-process, no external dependency).
+# Test:          test adapter.
+# Production:    Redis adapter — required for multi-worker deployments.
+#                Redis is also used for Mozo::Counter (replacing DrbCounter).
 #
 development:
   adapter: async
@@ -12,7 +12,6 @@ test:
   adapter: test
 
 production:
-  adapter: async
-  # adapter: redis
-  # url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
-  # channel_prefix: mozo_backend_production
+  adapter: redis
+  url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
+  channel_prefix: mozo_backend