fix: remove nonexistent change column from price collector OHLCV mapping

pykrx get_market_ohlcv returns 6 data columns (시가/고가/저가/종가/거래량/거래대금),
not 7. The 등락률 (change) column does not exist, causing a length mismatch error.

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

backend/app/services/collectors/price_collector.py

@@ -78,10 +78,10 @@ class PriceCollector(BaseCollector):
 
                 df = df.reset_index()
                 df.columns = ["date", "open", "high", "low", "close", "volume",
-                             "value", "change"]
+                             "value"]
 
                 # Validate column count
-                expected_cols = 8  # date + 7 data columns
+                expected_cols = 7  # date + 6 data columns
                 if len(df.columns) < expected_cols:
                     logger.warning(f"Unexpected column count for {ticker}: {len(df.columns)}")
                     continue