Add multi-stage Dockerfile, docker-compose, and fix insert conflict

Rebuild Dockerfile with 3 stages (CSS, uberjar, runtime) on port 3030.
Add docker-compose.yml loading .env. Fix insert-price! ON CONFLICT to
target (time_dk, price_area) matching the actual unique index.

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

resources/queries.sql

@@ -2,7 +2,7 @@
 -- :doc Insert a day-ahead price record, skip if already exists
 INSERT INTO day_ahead_prices (time_utc, time_dk, price_area, price_dkk, price_eur)
 VALUES (:time-utc, :time-dk, :price-area, :price-dkk, :price-eur)
-ON CONFLICT (time_utc, price_area) DO NOTHING;
+ON CONFLICT (time_dk, price_area) DO NOTHING;
 
 -- :name get-prices-for-date :? :*
 -- :doc Get all prices for a given date (time_dk local date) and price area