- Add is_topic boolean field to HadisCategory model with indexing
- Enforce validation rules in clean():
* Topics cannot have children and must define a source_type
* Structural categories cannot contain hadiths and have source_type=None
* Parents cannot be topics
* Hadiths can only be attached to topics (is_topic=True)
- Update admin serializers and views to support is_topic filtering and mapping
- Expose is_topic in public category serializers (SimpleCategory, HadisCategorySelectSerializer, CategorySerializer)
- Include migration 0051_add_category_is_topic