Skip existing prices on insert and add unique index on (time_dk, price_area)

Changed upsert to INSERT ON CONFLICT DO NOTHING so existing prices are
never overwritten. Added unique index on (time_dk, price_area) to enforce
no duplicate quarter-hour entries per area.

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

resources/queries.sql

@@ -1,11 +1,8 @@
--- :name upsert-price! :! :n
--- :doc Upsert a day-ahead price record
+-- :name insert-price! :! :n
+-- :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 UPDATE
-SET time_dk = EXCLUDED.time_dk,
-    price_dkk = EXCLUDED.price_dkk,
-    price_eur = EXCLUDED.price_eur;
+ON CONFLICT (time_utc, price_area) DO NOTHING;
 
 -- :name get-prices-for-date :? :*
 -- :doc Get all prices for a given date (time_dk local date) and price area