Captures the reasoning behind persisting two scalar columns on
documents rather than deriving aspect client-side or standing up a
thumbnail_metadata table. Also documents the 1.1 landscape threshold,
the null-during-rollout state, and the ordering invariants inside
ThumbnailService.generate().
Refs #305
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
