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>

docker-compose.yml

@@ -4,10 +4,13 @@ services:
     restart: unless-stopped
     ports:
       - "4101:4101"
+    depends_on:
+      - frankfurter
     environment:
       BTCDATA_PORT: "4101"
       JDBC_URL: "jdbc:postgresql://postgres:5432/btcprod?user=postgres&password=ratata,123"
       CORS_ORIGIN: "http://localhost:4041"
+      FRANKFURTER_URL: "http://frankfurter:8080"
     extra_hosts:
       - "postgres:host-gateway"