Fix Binance boot using OKX symbols after restart.

Startup ignored DB exchange choice and kept OKX ATM ids under a Binance health label, so option bid/ask stayed empty.

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

backend/app/main.py

@@ -30,9 +30,13 @@ def resolve_frontend_dist() -> Path:
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    settings = get_settings()
     db = Database()
     set_db(db)
+    # 必须在 set_db 之后读 DB 覆盖，否则会按 .env 默认 okx 起会话，
+    # 而 health 显示 DB 里的 binance → 合约号/盘口错乱（期权一直 -/-）
+    from .exchange.runtime import load_runtime_settings
+
+    settings = load_runtime_settings()
     engine = StrategyEngine()
     set_engine(engine)
     engine.ensure_loop()