Implement Stellar Route Fee Trend Analyzer

# 🧠 Concept
Analyze route fee trends over time.

## ⚠️ Problem
Fee volatility is difficult to understand.

## 📁 Implementation Scope
- `src/analytics/fees/stellar/`

## ✅ Requirements
- Track historical fees
- Generate trend reports
- Detect fee anomalies

## 🎯 Acceptance Criteria
- [ ] Fee trend analyzer implemented
- [ ] Trend reports generated