feat: Implement robust exchange rate fetching and aggregation with multiple providers, caching, and historical storage, along with supporting infrastructure.

[chibokaxavier]

Overview
This PR implements a resilient Exchange Rate Service designed to fetch, aggregate, and cache crypto-to-fiat rates. The system utilizes a multi-source strategy (Coinbase, Binance, and CoinGecko) to ensure high availability and data accuracy.

🛠️ Key Features
Multi-Source Aggregation: Fetches data from Coinbase and Binance, using a Weighted Average algorithm to calculate the final rate.

Resilience & Fallbacks: CoinGecko acts as a secondary fallback if primary sources fail.

An API Fallback Chain ensures that a single provider outage doesn't break the service.

Data Quality: Includes Outlier Detection to filter out "flash crash" or "bad data" spikes and a Confidence Scoring system for each rate.

Performance & Caching: * Implemented Redis caching for sub-millisecond lookups.

Integrated Staleness Detection to ensure users never receive expired data.

Infrastructure: * Cron Job: Automated updates every 60 seconds.

Persistence: Historical rate storage for auditing and trend analysis.

📊 Rate Calculation Logic
The service doesn't just pick a number; it follows this flow:

Fetch from all active providers.

Filter out outliers (values that deviate significantly from the median).

Calculate a weighted average based on source reliability.

Apply Spread for business logic.

Cache the result in Redis.

🧪 Testing & Verification
Unit Tests: All API responses are mocked to test edge cases and network timeouts.

Manual Verification: * Verified Cron logs for 60-second intervals.

Simulated API failures by blocking provider endpoints to confirm fallback to CoinGecko.

Checked Redis keys to confirm TTL and sub-millisecond response times.

📝 Definition of Done (Acceptance Criteria Check)
[x] Rates update every 60s.

[x] Graceful handling of source failures.

[x] Alerts triggered for stale rates.

[x] Sub-millisecond lookup via Redis.

Fixes #26