Require painting thumb regen on every picture change and document the invariant.

Fix disk sync, ensure, and preload paths that previously reused the full image as a thumb.

Co-authored-by: Cursor <cursoragent@cursor.com>

Documentation/Plans.md

@@ -1,5 +1,11 @@
 this file contains draft for future releases and features
 
+## Standing requirements (do not regress)
+
+- **Painting thumbnails:** any create/replace/clear of a painting’s full picture must regenerate or remove its dedicated `paintings/thumbs/` file — never use the full image or a remote thumb URL as `thumbnail_path`. See [data-and-images.md — Painting thumbnail invariant](data-and-images.md#painting-thumbnail-invariant).
+
+## Feature backlog
+
 1. ~~Multi language support, russian version at least~~ — done: UI i18n (EN/RU) + `entity_translations` DB + curator Translations tool — [i18n-russian.md](i18n-russian.md)
 2. ~~tool to manage links (influence/influenced by ) import csv's ( define format), edit ,add, delete, visualize, map to pictures/ entities~~ — done: curator Influences page (list CRUD + import wizard CSV/JSON/XLSX + neighborhood graph) — [influence-import.md](influence-import.md)
 3. tool to monitor/manage (plan actions) of curator actions, markers to check painting/text ?