Persist Frankfurter EUR/DKK rates in currencies table

Store daily exchange rates in PostgreSQL with EUR/DKK cross rate.
Seed rates atom from DB on startup so WebSocket works immediately
even if Frankfurter is unreachable.

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

resources/system.edn

@@ -61,7 +61,8 @@
  {:query-fn #ig/ref :db.sql/query-fn}
 
  :frankfurter/rates
- {:url #or [#env FRANKFURTER_URL "http://localhost:8080"]}
+ {:url      #or [#env FRANKFURTER_URL "http://localhost:8080"]
+  :query-fn #ig/ref :db.sql/query-fn}
 
  :strike/ticker
  {:query-fn        #ig/ref :db.sql/query-fn