CVE-2026-82394: Sulu: Fix authorization bypass when creating preview links

A missing authorization check on the preview link endpoint lets a backend user create a public, unauthenticated preview URL for content they are not allowed to see.

PreviewLinkController (and the underlying PreviewLinkManager::generate() / revoke()) never enforced a permission on the target resource. Any authenticated administration user could call the generate action for any page, article or snippet, including content in a webspace or area they have no VIEW rights on. The endpoint returns a public render URL that resolves the content solely by an opaque token, so the attacker (or anyone they share the link with) can then read the restricted content without authentication.

This affects installations that use Sulu role and webspace permissions to restrict who may see certain content. Exploitation requires an authenticated backend user and the id of the target resource.