Implement Stellar Account Permission Scanner

# 🧠 Concept  
Analyze Stellar account authorization configurations.

## ⚠️ Problem  
Improper account permissions may expose operational risk.

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

## ✅ Requirements  
- Detect signer thresholds  
- Validate authorization settings  

## 🎯 Acceptance Criteria  
- [ ] Permission scanner implemented  