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/migrations/20260214130000-add-unique-index-time-dk-price-area.up.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX idx_day_ahead_prices_time_dk_area ON day_ahead_prices (time_dk, price_area);