fix(rates): instantiate RateService singleton in AppState to enable cache (#36)

[ghzhost]

Overview

Fixes #36 by instantiating RateService once at startup and storing it in AppState, allowing GET /api/rates to properly share and leverage the in-memory TTL cache across requests.

Changes

1. AppState Singleton: Added pub rate_service: services::rate::RateService to AppState and initialized it once during startup in src/main.rs using the configured horizon_url and rate_cache_ttl_secs.
2. Route Handler: Updated src/routes/rates.rs to call state.rate_service.fetch_rate(...) directly instead of allocating a fresh RateService (with a discarded empty cache) on every request.
3. Unit Tests: Added wiremock-backed tests in src/services/rate.rs verifying that cache hits occur within the configured TTL window without repeating outbound calls to Horizon.