fix: correct pykrx ETF module import path and method call

The pykrx library uses 'etx' not 'etf' as the module directory name,
and fetch() is the proper method that returns a DataFrame.

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

backend/app/services/collectors/etf_collector.py

@@ -4,7 +4,7 @@ ETF master data collector from KRX.
 import logging
 
 import pandas as pd
-from pykrx.website.krx.etf.core import ETF_전종목기본종목
+from pykrx.website.krx.etx.core import ETF_전종목기본종목
 
 from sqlalchemy.orm import Session
 from sqlalchemy.dialects.postgresql import insert
@@ -42,7 +42,7 @@ class ETFCollector(BaseCollector):
 
     def collect(self) -> int:
         """Collect ETF master data."""
-        df = ETF_전종목기본종목().read()
+        df = ETF_전종목기본종목().fetch()
 
         if df.empty:
             logger.warning("No ETF data returned from KRX.")