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>

src/clj/pmagnus/elprice/web/controllers/prices.clj

@@ -57,10 +57,10 @@
      (.replace ^String s "T" " "))))
 
 (defn save-records!
-  "Upsert price records into the database."
+  "Insert price records into the database, skipping existing ones."
   [query-fn records]
   (doseq [rec records]
-    (query-fn :upsert-price!
+    (query-fn :insert-price!
               {:time-utc   (parse-timestamp (:TimeUTC rec))
                :time-dk    (parse-timestamp (:TimeDK rec))
                :price-area (:PriceArea rec)