[chenasraf]

• Threads have a Summarize menu with Summarize entire thread and Summarize unread posts
• Unread summaries cover replies since the last visit, with the opening post as context; the option is disabled when there is nothing unread
• The menu appears only to signed-in people, and only when a Nextcloud summary provider (e.g. Assistant, OpenAI integration) is available
• The summary appears above the posts, is labelled AI-generated, and is visible only to the person who requested it; it is not saved as a post
• Summaries respect category view permissions
• Summarizing an unchanged thread again reuses the earlier result
• Administration can turn summaries off under Artificial intelligence in the forum settings

Closes #344