fix: use isolated margin to prevent cross-position liquidation risk

Add set_margin_type("ISOLATED") call before each position entry.
With cross margin, a single bad trade's loss draws from the entire
account balance. Isolated margin caps loss to the position's allocated
margin only.

Binance returns -4046 if already ISOLATED, which is silently ignored.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bot.py

@@ -437,6 +437,7 @@ class TradingBot:
                 return
 
             side = "BUY" if signal == "LONG" else "SELL"
+            await self.exchange.set_margin_type("ISOLATED")
             await self.exchange.set_leverage(self.config.leverage)
             await self.exchange.place_order(side=side, quantity=quantity)
 

src/exchange.py

@@ -107,6 +107,21 @@ class BinanceFuturesClient:
             ),
         )
 
+    async def set_margin_type(self, margin_type: str = "ISOLATED") -> None:
+        """마진 타입을 변경한다. 이미 동일 타입이면 무시."""
+        try:
+            await self._run_api(
+                lambda: self.client.futures_change_margin_type(
+                    symbol=self.symbol, marginType=margin_type
+                ),
+            )
+            logger.info(f"[{self.symbol}] 마진 타입 변경: {margin_type}")
+        except BinanceAPIException as e:
+            if e.code == -4046:  # "No need to change margin type."
+                logger.debug(f"[{self.symbol}] 마진 타입 이미 {margin_type}")
+            else:
+                raise
+
     async def get_balance(self) -> float:
         balances = await self._run_api(self.client.futures_account_balance)
         for b in balances: