Lychee: unauthenticated photo starring reveals existence of private photos when photos_star_visibility=anonymous

Lychee is a free, open-source photo-management tool. Prior to 7.6.1, when photos_star_visibility is set to anonymous, POST /api/v2/Photo::highlight can fetch and modify a photo without confirming that the caller can see it. BaseApiRequest::validateResolved() invokes SetPhotosHighlightedRequest::processValidatedValues() before authorization, and PhotoPolicy::canHighlight() returns true for anonymous callers without a canSee check. An unauthenticated attacker who can guess photo identifiers can distinguish existing private photos through differing success and not-found responses and can change their is_highlighted state, including for photos in private or password-protected albums. The issue is limited to installations using the non-default anonymous star-visibility setting. This issue is fixed in version 7.6.1.