Media Manager: add Document privacy controls

[stefanmuellerdo]

Description

Add configurable privacy settings to Media Manager to give merchants more control over media file access.

Reason

Currently all Media Manager files are public by default afer upload, but some use cases require restricted access, matching the same level of restricted access as available on Page Builder story etc.

Example use case: A promotional discount on a product configured to be public next week contains a picture with the discount. The picture URL could be discovered via scraping, revealing the promotion too early.

Proposed Changes

• Add privacy toggle in Media Manager UI (public/private)
• Implement basic access control matching product visibility rules
• Store private files in protected location
• Add validation in Page Builder to check for restricted media
• Add visual indicator for private media files in Media Manager