feat: replace mock APY sources with real protocol fetchers (closes #157)

[BernardOnuh]

Description

This PR replaces the mocked APY data in the scanner with real data fetchers from three Stellar protocols.

Closes #157

Changes Made

• ✅ Added fetchWithRetry utility with timeout, retry, and circuit breaker support
• ✅ Implemented real fetchBlendApy() using Blend Capital API
• ✅ Implemented real fetchStellarDexApy() using Stellar Horizon API
• ✅ Implemented real fetchLumaApy() using Luma Finance API
• ✅ Added metrics tracking: fetch duration, failures per source, stale-rate detection
• ✅ Added persistence to ProtocolRate table with raw response data
• ✅ Added TVL filtering (minimum 10,000 USDC)
• ✅ Added comprehensive error handling and logging

Implementation Details

• Blend: Fetches USDC reserve supply APY from Blend Capital API
• Stellar DEX: Aggregates weighted average fee APY from liquidity pools
• Luma: Fetches USDC APY rates from Luma Finance API

Testing

• All fetchers include timeout protection (5s)
• Retry logic with exponential backoff (3 retries)
• Circuit breaker prevents cascading failures
• Graceful degradation if any protocol fails

Environment Variables Required

STELLAR_NETWORK=mainnet|testnet|futurenet
HORIZON_URL=https://horizon.stellar.org  # optional
BLEND_POOL_ID=GBUQWP3BOUZX34PISXEAMBNIZJLNCLVNX77MHAHVXHVVB4CMYAOK6BAC  # optional
LUMA_API_URL=https://api.luma.finance  # optional
USDC_ISSUER=GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN  # optional