Add updated_since filter to GET /v1/posts

Description: As an API consumer, I want a native updated_since query parameter on GET /v1/posts so I can retrieve only posts updated since a specified timestamp (e.g., last 5 days) without over-fetching. Problem: There is no way to filter by the native updated field. The advanced parameter only filters on custom metadata. The current workaround is to sort by updated and paginate client-side until reaching older posts, which causes unnecessary data transfer and extra requests. Benefit: Server-side filtering by last updated reduces bandwidth and client complexity, speeds up incremental syncs, and lowers API and client load by returning only relevant, recently changed posts.