修复主力合约识别：按持仓量判定并移除当月占位

Co-authored-by: Cursor <cursoragent@cursor.com>

app.py

@@ -17,7 +17,7 @@ from flask import (
 )
 from werkzeug.security import check_password_hash, generate_password_hash
 
-from symbols import search_symbols, ths_to_codes, list_main_contracts_grouped
+from symbols import search_symbols, ths_to_codes, list_main_contracts_grouped, refresh_main_index
 from contract_specs import calc_position_metrics
 from fee_specs import (
     calc_fee_breakdown,
@@ -580,6 +580,7 @@ def start_background_threads():
         target=lambda: kline_hub.worker_loop(DB_PATH, build_market_quote_payload),
         daemon=True,
     ).start()
+    threading.Thread(target=refresh_main_index, daemon=True).start()
 
 
 start_background_threads()