FEATURE: per-topic unsubscribe option in emails

[akshayutture-augment]

Test 2

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Replicated from ai-code-review-evaluation/discourse-cursor#2

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Note

Adds a per-topic unsubscribe endpoint and UI, updates email templates/links to point to it, and wires supporting routes, model helper, and tests.

• Email/Notifications:
  • Add per-topic unsubscribe link support; pass unsubscribe_url and render %{unsubscribe_link} in notification.html.erb and text bodies via MessageBuilder.
  • Set unsubscribe_url to post.topic.unsubscribe_url in UserNotifications#send_notification_email.
  • Include List-Unsubscribe header and keep respond instructions formatting.
  • Update specs to assert presence of unsubscribe_url handling.
• Backend:
  • New TopicsController#unsubscribe action to downgrade TopicUser.notification_level (to regular or muted) and render topic view; allow non-XHR.
  • Add routes GET /t/:slug/:topic_id/unsubscribe and fallback GET /t/:topic_id/unsubscribe.
  • Add Topic#unsubscribe_url helper.
• Frontend:
  • New route topicUnsubscribe, controller topic-unsubscribe.js.es6, view, and template templates/topic/unsubscribe.hbs with topic-notifications-button.
  • Load topic via PostStream.loadTopicView, hide notificationReasonText, and show footer.
  • Styles for .topic-unsubscribe .notification-options dropdown.
  • dropdown-button only renders title <h4> when title is present; other minor ES6/const cleanups.
• i18n:
  • Add client keys under topic.unsubscribe.* and server unsubscribe_link text including per-topic link.

^{Written by Cursor Bugbot for commit 6669a2d. Configure here.}

[akshayutture-augment]

cursor review

[cursor]

Bug: Unsubscribe fails for missing topic user.

The unsubscribe method calls TopicUser.find_by which returns nil when no TopicUser record exists for the user-topic combination. The code then attempts to access tu.notification_level and call tu.save! without checking if tu is nil, causing a NoMethodError when users who haven't interacted with the topic try to unsubscribe.