Add ActionCable connection identification

[dadachi]

Summary

• Add identified_by :current_shopkeeper, :current_account to ActionCable connection using warden session lookup
• Anonymous WebSocket connections remain allowed for public display pages (no reject_unauthorized_connection)
• Document convention for future authenticated channels to reject in #subscribed
• Add connection tests for both anonymous and authenticated scenarios

Test plan

• bin/rails test test/channels/application_cable/connection_test.rb — new tests pass
• bin/rubocop — no style violations
• bin/brakeman — no security warnings
• bin/rails test — full test suite passes (336 tests, 0 failures)
• Verify display pages still load and update via Turbo Streams in development

🤖 Generated with Claude Code