Implement Stellar Event Streaming Integration

# 🧠 Concept  
Stream live Stellar contract events into GasGuard.

## ⚠️ Problem  
Real-time event analysis is unavailable.

## 📁 Implementation Scope  
- `src/streaming/stellar/`

## ✅ Requirements  
- Subscribe to event streams  
- Process incoming events  

## 🎯 Acceptance Criteria  
- [ ] Event streaming implemented  
- [ ] Real-time processing functional  