Auto-hide feedback posts submitted before a cutoff date

Allow admins to set a cutoff date so that any feedback post submitted before that date is automatically hidden from the public Feedback Portal list (while still being manageable in the admin dashboard).

Use case: teams with a large, aging backlog want new users to see only recent, relevant feedback rather than scrolling through years of stale requests. This should apply automatically to anything older than the configured date, rather than requiring posts to be hidden one by one.

Requested by Marta Ondresik at Digital Science during a demo call.