Content can go live pointing at a soft-deleted asset or entry #2665

Part of #2427 (F-MED). Found by the review of #2555.

Checks skip an asset the library cannot find, and a soft-deleted asset is not found. Publishing checks only whether the item being published is deleted, not what it references. So content that shows a deleted image passes the alt text check and can publish, and a restore later brings the image back live without alt text. Deleted entries that other entries reference behave the same way today.

Steps:

1. A page shows a published image without alt text, and its set fails the alt text check.
2. A person deletes the image. The page's set now passes the check and publishes.
3. The person restores the image. The page is live with an image that has no alt text.

Scope

• A check run or a publish treats a reference to a soft-deleted asset or entry as a problem, so content cannot go live pointing at a deleted item. A finding or a publish failure should name the item and say to restore it or remove the reference.

Out of scope

• Purging deleted items.

Acceptance

• A page whose media field names a deleted asset fails its set's checks, or fails to publish, with a message that names the asset.
• After the asset is restored, the alt text check reads it as before.
• Tested on SQLite and PostgreSQL.