feat(canvas): 画布资源 - 缩略图字段与缓存加载支持

backend/internal/repository/repository.go

@@ -1008,6 +1008,14 @@ func (r *Repository) SaveResource(resource *model.Resource) error {
 	return r.db.Save(resource).Error
 }
 
+// UpdateResourceThumbnail only updates a still-ready row and cannot resurrect a deleted resource.
+func (r *Repository) UpdateResourceThumbnail(userID string, id string, values map[string]any) (bool, error) {
+	result := r.db.Model(&model.Resource{}).
+		Where("id = ? AND user_id = ? AND status = ?", id, userID, model.ResourceStatusReady).
+		Updates(values)
+	return result.RowsAffected == 1, result.Error
+}
+
 func (r *Repository) ResourceByUploadKey(userID string, uploadKey string) (*model.Resource, error) {
 	var resource model.Resource
 	if err := r.db.First(&resource, "user_id = ? AND upload_key = ?", userID, uploadKey).Error; err != nil {