Categories/Tags

- Allow posts to be categorized or tagged:
- Create a separate Category or Tag model.
- Associate posts with categories or tags using relationships.
- Add filtering by category or tag on the homepage.

Look at the existing Post model in models.py to understand the database schema. Create new Category or Tag models with appropriate relationships (many-to-many or foreign key). Update the post creation and editing forms and views to handle categories/tags. Modify the homepage view and template to add filtering by category or tag, likely by adding query parameters and database filters.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.