US-44 — Show presence information for an imported article

## User story
*As a Dispatcher user, I want to see who else is viewing an imported Guardian article so that I know when colleagues may be working with the same content.*

## Acceptance scenarios

```gherkin
Feature: Show presence information for an imported article

Background:
Given the user has access to Dispatcher

Scenario: Do not show presence before an article is imported
Given the user has not imported an article
When the user opens the notification composition page
Then the presence indicator should not be displayed

Scenario: Show users viewing an imported article
Given the user has imported a Guardian article
And one or more Dispatcher users are viewing the same article
When the article is displayed
Then a presence indicator should be displayed beneath the article
And the indicator should show the users viewing that article

Scenario: Update presence information
Given the user has imported a Guardian article
And the presence indicator is displayed
When another Dispatcher user starts or stops viewing the same article
Then the presence indicator should update to reflect the current viewers

Scenario: Do not show an empty presence indicator
Given the user has imported a Guardian article
And no users are reported as viewing that article
When the article is displayed
Then the presence indicator should not be displayed

Scenario Outline: Show presence for an imported article across notification channels
Given the user is creating a newsletter email OR app alert
When the user imports a Guardian article
Then the imported article should be displayed
And the presence indicator should appear beneath the article
And the notification should use the imported article when sent
```

Start by reviewing linked pull request #575 and tracing the notification composition page and imported Guardian article flow. Use the listed acceptance scenarios as the completion checklist: presence appears only for imported articles with viewers, updates as viewers change, remains hidden when empty, and works for newsletter email and app alert channels.

Written by the indexing model from the issue text.