NoMethodError: undefined method 'submissions' for nil (NoMethodError)

## Error

```
undefined method 'submissions' for nil (NoMethodError)

solution = current_user.submissions.find_by!(uuid: params[:uuid])
^^^^^^^^^^^^
```

## Stack trace

```
app/channels/latest_iteration_status_channel.rb:3 in subscribed
action_cable/channel/base.rb:184 in block in ActionCable::Channel::Base#subscribe_to_channel
active_support/callbacks.rb:121 in block in ActiveSupport::Callbacks#run_callbacks
sentry/rails/action_cable.rb:26 in block in Sentry::Rails::ActionCableExtensions::ErrorHandler.capture
sentry/hub.rb:89 in with_scope
sentry-ruby.rb:399 in Sentry.with_scope
sentry/rails/action_cable.rb:18 in Sentry::Rails::ActionCableExtensions::ErrorHandler.capture
sentry/rails/action_cable.rb:89 in sentry_capture
sentry/rails/action_cable.rb:79 in block (2 levels) in Sentry::Rails::ActionCableExtensions::Channel::Subscriptions.included
active_support/callbacks.rb:130 in instance_exec
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [NoMethodError: undefined method 'submissions' for nil (NoMethodError)](https://thalamus-ai.sentry.io/issues/143094710/)
- **Culprit**: LatestIterationStatusChannel#subscribed
- **First seen:** Aug 26, 2026

Start at app/channels/latest_iteration_status_channel.rb:3 and inspect how current_user is established for subscriptions. Reproduce the Sentry error through the channel subscription path and locate any existing channel tests or authentication setup. Done means the nil-user path has defined behavior and is covered by a regression test.

Written by the indexing model from the issue text.