Add Frankfurter EUR/DKK rate polling and include rates in WebSocket stream

Polls self-hosted Frankfurter API hourly for USD-based EUR and DKK exchange
rates. Both the WebSocket HTML fragments and the /api/price/latest JSON
endpoint now include the currency rates alongside BTC price.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -44,8 +44,8 @@ Docker runs alongside local dev on different ports and databases:
 ## API Endpoints
 
 - `GET /api/health` — Health check
-- `GET /api/price/ws` — WebSocket endpoint for live HTML price fragments (HTMX-compatible)
-- `GET /api/price/latest` — Latest price as JSON
+- `GET /api/price/ws` — WebSocket endpoint for live HTML price fragments with EUR/DKK rates (HTMX-compatible)
+- `GET /api/price/latest` — Latest price + EUR/DKK rates as JSON
 
 ## Source Layout
 
@@ -55,6 +55,8 @@ src/clj/pmagnus/btcdata/
 ├── config.clj            # System config loader
 ├── ws/
 │   └── binance.clj       # Binance WebSocket client
+├── frankfurter/
+│   └── rates.clj         # EUR/DKK exchange rate poller
 ├── kraken/
 │   ├── ohlc.clj          # Hourly OHLC poller
 │   └── ohlc_daily.clj    # Daily OHLC poller