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>

resources/system.edn

@@ -44,9 +44,10 @@
   :migrate-on-init? true}
 
  :reitit.routes/api
- {:base-path "/api"
-  :query-fn  #ig/ref :db.sql/query-fn
-  :binance   #ig/ref :ws/binance}
+ {:base-path    "/api"
+  :query-fn     #ig/ref :db.sql/query-fn
+  :binance      #ig/ref :ws/binance
+  :frankfurter  #ig/ref :frankfurter/rates}
 
  :ws/binance
  {:query-fn #ig/ref :db.sql/query-fn
@@ -56,4 +57,7 @@
  {:query-fn #ig/ref :db.sql/query-fn}
 
  :kraken/ohlc-day
- {:query-fn #ig/ref :db.sql/query-fn}}
+ {:query-fn #ig/ref :db.sql/query-fn}
+
+ :frankfurter/rates
+ {:url #or [#env FRANKFURTER_URL "http://localhost:8080"]}}