Fixes: 1) Correct default value in `recently_viewed` property to handle cache miss;
Extra: 1) Inline simplification of serializer logic for recently viewed products.
Fixes: 1) Remove 'recently_viewed' ManyToManyField from User model and related migration; 2) Correct handling of product addition in 'recently_viewed' logic.
Extra: Refactor 'recently_viewed' to use cache instead of database field; cleanup unused imports and model attributes.
