This PR implements Redis-based response caching for frequently accessed static data endpoints to improve API performance and reduce backend load.

Key Changes:

• Added Redis cache middleware for /currencies, /institutions/:currency_code, and /pubkey endpoints
• Implemented cache warming on server startup
• Added cache hit/miss metrics using Prometheus
• Added proper cache headers (Cache-Control, ETag) and handling
• Implemented stale-while-revalidate pattern for high availability

Cache Configuration:

• Currencies: 24 hours
• Institutions by currency: 24 hours
• Public key: 365 days

Cache Keys:

• api:currencies:list
• api:institutions:{currency_code}
• api:aggregator:pubkey

Testing:

• Added comprehensive test suite for cache middleware
• Includes tests for cache hits, misses, and revalidation
• Added benchmarks for cache performance

closes #381