Add GET /api/currencies/:date endpoint for historical EUR/DKK rates

Returns rates for a given date: today uses latest DB row, past dates
check DB first then fetch from Frankfurter and persist for caching.
Future dates and invalid formats return 400.

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

resources/queries.sql

@@ -60,4 +60,8 @@ SET eur = EXCLUDED.eur, dkk = EXCLUDED.dkk, eur_dkk = EXCLUDED.eur_dkk, updated_
 
 -- :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
+SELECT rate_date, eur, dkk, eur_dkk, updated_at FROM currencies ORDER BY rate_date DESC LIMIT 1
+
+-- :name get-currency-rates-by-date :? :1
+-- :doc Get currency rates for a specific date
+SELECT rate_date, eur, dkk, eur_dkk, updated_at FROM currencies WHERE rate_date = :rate-date