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/migrations/20260306200000-create-currencies.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE currencies (
+  rate_date DATE PRIMARY KEY,
+  eur NUMERIC(12,6) NOT NULL,
+  dkk NUMERIC(12,6) NOT NULL,
+  eur_dkk NUMERIC(12,6) NOT NULL,
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);