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/queries.sql

@@ -50,3 +50,14 @@ INSERT INTO strike_price (rates, quote_sats) VALUES (:rates, :quote-sats)
 -- :name get-latest-strike-price :? :1
 -- :doc Get the most recent Strike ticker snapshot
 SELECT rates, recorded_at FROM strike_price ORDER BY id DESC LIMIT 1
+
+-- :name upsert-currency-rates! :! :n
+-- :doc Upsert daily currency rates from Frankfurter
+INSERT INTO currencies (rate_date, eur, dkk, eur_dkk, updated_at)
+VALUES (:rate-date, :eur, :dkk, :eur-dkk, NOW())
+ON CONFLICT (rate_date) DO UPDATE
+SET eur = EXCLUDED.eur, dkk = EXCLUDED.dkk, eur_dkk = EXCLUDED.eur_dkk, updated_at = NOW()
+
+-- :name get-latest-currency-rates :? :1
+-- :doc Get the most recent currency rates
+SELECT eur, dkk, eur_dkk, updated_at FROM currencies ORDER BY rate_date DESC LIMIT 1