fix(media): bound temporary link preview retention #196

URL resolution stores managed thumbnail and site-icon media before content is published. Abandoned compositions need automatic reclamation, while saved snapshots require durable media references. Preview dismissal also needs to end with the composition that owns it.

Expected behavior:

• Reclaim only captured link resources after at least 24 continuous hours without a content or retained-draft reference; exclude ordinary uploads, including deduplicated uploads.
• Protect retained browser drafts before confirming a save, preserve their card snapshots, and release or expire their temporary references safely.
• Reset obsolete dismissal state when a composition is cleared or completed, without reviving a card explicitly removed from the current content.

Reference changes and cleanup must share the existing media coordination boundary, and published content must remain protected.

Implementation

#198