[codex] Add KIS proxy cache and outbound rate limiter

[smallfish06]

Summary

• add an in-process KIS proxy cache with endpoint-aware TTLs, singleflight request coalescing, and stale fallback on upstream errors
• add an outbound KIS proxy rate limiter so cache misses and uncached proxy calls are throttled before reaching KIS
• expose KIS proxy cache/rate-limit options through the public server API and YAML config
• keep trading/order/auth endpoints out of cache while still applying outbound throttling

Why

KIS upstream rate limits were surfacing under repeated requester traffic. API-side retries and DB caching reduce some load, but the durable bottleneck belongs inside the requester/krsec layer immediately before CallEndpoint, after cache hits have been resolved.

Validation

• go test ./pkg/config ./internal/server ./pkg/server
• go test ./...