Posts order not updating after unpinning a post

[eemaanamir]

When a post is unpinned after being pinned, it remains at the top of the tab even after a page reload.

How to reproduce:

• Navigate to an old post in your course
• Pin the post so that the post appears at the top of the tab
• Unpin the post
• Hard reload the page
• The post will remain at the top of the list despite the value of pinned set to false in the returned data from the API call

Note:
This issue did not exist in forum v1 before the following PR was merged:
openedx/openedx-platform#36664

[taimoor-ahmed-1]

@eemaanamir in the test environment is the discussions mfe enabled?

[taimoor-ahmed-1]

The issue is that last_activity_at gets updated when pinning/unpinning threads, which affects the sorting order. When a thread is unpinned, the updated_at field (which influences last_activity_at sorting) gets updated to the current timestamp, causing the recently unpinned thread to appear at the top of the list even though pinned=False. This happens because both backends update timestamps during pin/unpin operations - MySQL via Django's auto_now=True on updated_at, and MongoDB by explicitly setting updated_at = datetime.now(). The fix is to prevent timestamp updates during pin/unpin operations by only updating the pinned field.