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/initializers/mozo_settings.rb

@@ -8,12 +8,12 @@ else
   Mozo.user_url     = 'https://user.mozo.bar'
 end
 
+# Broadcaster: swap Faye ↔ ActionCable
+#   Mozo.broadcaster = Mozo::Broadcaster::Faye.new        # current (HTTP POST to Faye)
+#   Mozo.broadcaster = Mozo::Broadcaster::ActionCable.new  # new (in-process async)
 Mozo.broadcaster = Mozo::Broadcaster::Faye.new
 
-# use the connection from couchbase-structures/documents
-# will be overwritten in the specs since flushing the real
-# thing is difficult
-# Mozo::Counter.connection = $cb unless Rails.env.test?
-
-# Use the Drb counter
+# Counter: swap DrbCounter ↔ Redis
+#   Mozo::Counter.connection = Mozo::DrbCounter.object          # current (DRb in-memory)
+#   Mozo::Counter.connection = Mozo::Counter::Redis.new         # new (persistent, multi-process)
 Mozo::Counter.connection = Mozo::DrbCounter.object unless Rails.env.test?