Step 6 migration: add JSON backup and restore flows

README.md

@@ -140,6 +140,10 @@ git pull
   - POST `/api/catalog/import`
   - bouton UI `Migrer localStorage vers DB`
   - deduplication: `console + titre + annee`
+- Etape 6: backup/restauration locale JSON
+  - GET `/api/backup/export`
+  - POST `/api/backup/restore` (modes `merge` ou `replace`)
+  - snapshot auto en base avant restore `replace` (`backup_snapshots`)
 
 ## Licence