fix: Remove app/channels directory

The app/channels directory contains ActionCable boilerplate that references
ActionCable::Channel::Base, but we excluded ActionCable from loading in
config/application.rb.

Since we're not using WebSockets/ActionCable for this JSON API, removing
the entire directory resolves the initialization errors.

Changes:
- Removed app/channels/ directory

This completes the ActionCable removal.

Author: teetangh