src/rates.rs adds an in-memory rate cache (60s TTL) with a 3-source
fallback chain. AppState gains `rates: Arc<RateCache>`. Manual pins
via the settings table override the chain — used by tests for
deterministic conversions and by operators during maintenance
windows.
Admin endpoints:
- GET /v1/admin/rates: cache snapshot
- POST /v1/admin/rates/refresh: force re-fetch (audit-logged)
Two new tests (network-free, manual-pin path):
- rate_cache_honors_manual_pin_from_settings
- admin_rates_endpoint_reflects_manual_pin
Test count: 36 (was 34).
Grant