Implement Soroban Scan History Explorer

# 🧠 Concept  
Allow users to browse historical scans.

## ⚠️ Problem  
Past analysis results are difficult to review.

## 📁 Implementation Scope  
- `src/history/scans/stellar/`

## ✅ Requirements  
- Store scan metadata  
- Support filtering and lookup  

## 🎯 Acceptance Criteria  
- [ ] Scan history explorer implemented  