Astra's review of #366: isAssetShownOnLivePage read every reference to the
picture, and the trash can hold any number of pages that showed it. A new
index ending in pageId answers "does this page show it" in one read, so the
check reads one row per live page and never the trash. Additive index; no
data or client change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>