Create Security Case Management Entity

# 🧠 Concept
Track investigations independently of incidents.

## ⚠️ Problem
Complex investigations require structured case tracking.

## 📁 Implementation Scope
- prisma/schema.prisma

## ✅ Requirements
- Case model
- Ownership tracking
- Status tracking

## 🎯 Acceptance Criteria
- [ ] Case entity created
- [ ] Migration generated
- [ ] Relationships configured