feat(api): add TTL cache for escrow read endpoint

[okekefrancis112]

Summary

Closes #27

• Add MemoryCacheStore abstraction (src/services/cacheStore.js) backed by native Map with TTL-based lazy eviction, designed for future Redis swap-in
• Add cacheResponse Express middleware (src/middleware/cache.js) that intercepts res.json() to cache 2xx responses, sets X-Cache: HIT/MISS headers, and transparently falls through on cache errors
• Add cache config (src/config/cache.js) parsing ESCROW_CACHE_TTL_SECONDS env var (default: 30s)
• Wire middleware onto GET /api/escrow/:invoiceId route
• No new npm dependencies — uses native Map

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[mikewheeleer]

Resolve the conflicts & revert the changes in package-lock.json