Exchange Rate Cache with TTL Eviction

[Kingsman-99]

Description

src/currencyConverter.ts fetches exchange rates on every invocation, making repeated conversions in a single invoice rendering session unnecessarily expensive and slow. Rates fetched from external oracles rarely change within a minute, so a time-to-live cache should store fetched rates and serve subsequent requests from memory until the TTL expires, with background refresh to avoid cold-fetch latency on expiry.

Technical Context

Add src/rateCache.ts. Integrates with src/currencyConverter.ts and any price oracle adapter. Implements a Map<string, { rate: number; fetchedAt: number }> store keyed by "${fromAsset}:${toAsset}". Uses src/cache.ts eviction patterns. Background refresh implemented with a setInterval owned by RateCache lifecycle methods start() / stop(). Ties into src/gracefulShutdown.ts pattern from src/index.ts.

Acceptance Criteria

• RateCache.getRate(from, to): Promise<number> serves from cache when Date.now() - fetchedAt < ttlMs
• On first fetch or cache miss, calls the configured rate-oracle function and stores the result with a timestamp
• Background refresh fires at ttlMs * 0.8 to pre-warm the cache before expiry
• invalidate(from, to) removes a specific pair; invalidateAll() clears the entire cache
• Unit tests assert: cache hit (no oracle call), cache miss after TTL (oracle called once), background refresh (oracle called before expiry)
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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[Muneerat]

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[whisper011]

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