Add elprice table + JSON API (GET /api/elprice?date=...)

New `elprice` table with a simpler shape (time_dk, price_area,
price_dkk) backing a JSON endpoint that returns hourly DK1 + DK2
prices for any date. Cache-miss path fetches from the same
DayAheadPrices dataset as day_ahead_prices, upserts, and returns.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

resources/migrations/20260419010000-create-elprice.down.sql

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+DROP TABLE elprice;

resources/migrations/20260419010000-create-elprice.up.sql

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+CREATE TABLE elprice (
+  time_dk    TIMESTAMP     NOT NULL,
+  price_area VARCHAR(3)    NOT NULL,
+  price_dkk  NUMERIC(10,2) NOT NULL,
+  PRIMARY KEY (time_dk, price_area)
+);