Historical Exchange Rate Retrieval With Lru Caching

** - MCP server acting as an interface to the [Frankfurter API](https://frankfurter.dev/) for currency exchange data.

Unique: Implements LRU caching specifically for historical queries (separate from TTL cache for latest rates), recognizing that historical data is immutable and benefits from long-term caching. 1024-item capacity balances memory usage against typical agent workflows that may query 10-50 distinct historical periods.

vs others: More efficient than calling Frankfurter API repeatedly for the same historical dates; LRU strategy is appropriate for historical data (unlike TTL, which assumes data freshness matters) and avoids unbounded memory growth