Implement Model Property Caching with cached_property

Implement Model Property Caching with cached_property
Estimated effort: 2 points
Files: backend/apps/owasp/models/*.py, backend/apps/github/models/*.py

Replace @property with @cached_property for expensive operations that don't change during the object lifecycle:

Properties that make database queries
Properties with expensive calculations
Properties that call external APIs

**Are you going to work on implementing this?**

- [x ] Yes
- [] No