feat: implement tick-by-tick raw API response audit export

[feromoneth]

Closes #156

---

Issue Summary

Issue #156 requests high-granularity audit logging for institutional users — a complete trace of every raw API response (un-normalized) used in price derivation, enabling full transparency into how final prices are calculated.

---

Root Cause

The system normalizes and aggregates market data immediately, discarding raw provider payloads. Without persisted raw responses, audit trails cannot show the constituent API data that led to final price decisions, blocking institutional compliance and auditing requirements.

---

Fix Implemented

• Added new RawData Prisma model storing every successful external API response with provider name, endpoint, full JSON payload, and fetch timestamp
• Extended MarketRate interface to carry optional rawResponses array during fetch operations
• Updated all fetchers to capture payloads before normalization: CoinGecko (GHS, KES), VTpass, and ExchangeRate API
• Added guarded persistence path in MarketRateService that safely serializes and inserts raw data without blocking price updates on storage failure
• Indexed schema for efficient audit queries by currency, provider, and time range

---

Testing Performed

• Type-check passed on all modified files
• NGN fetcher test assertions validate raw response capture for all 5 providers
• Schema change follows existing Prisma model patterns in repository
• Persistence uses safe async error handling to protect existing fetch behavior

---

References

• Closes Implement "Tick-by-Tick" Export for Audit #156
• Updated: Prisma schema, MarketRateService, fetcher modules

[drips-wave]

@feromoneth Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits