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

Gemfile

@@ -7,6 +7,7 @@ source 'https://rubygems.org'
 gem 'rails', '~> 8.1.1'
 #gem 'rails', '7.1.1'
 gem 'rack-cors', require: 'rack/cors'
+gem 'redis', '~> 5.0'
 
 # Bundle edge Rails instead:
 # gem 'rails', git: 'git://github.com/rails/rails.git'