Harden LIVE opens with slot claim and exchange reconcile.

Prevent duplicate opens by atomically claiming an opening slot, verifying exchange perp is flat before live orders, setting leverage from ledger, and preferring exchange position size when closing perps.

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

backend/app/live/binance_trade.py

@@ -314,6 +314,39 @@ class BinanceTradeClient:
             return to_usdt(float(upl), "USDT")
         return 0.0
 
+    def get_perp_pos_sz(self, symbol: str, *, position_side: str | None = None) -> float | None:
+        """当前永续绝对持仓（ETH）。"""
+        try:
+            rows = self._signed(
+                self._fapi, "GET", "/fapi/v2/positionRisk", {"symbol": symbol}
+            )
+        except Exception as e:
+            logger.warning("binance get_perp_pos_sz failed: %s", e)
+            return None
+        if isinstance(rows, dict):
+            rows = [rows]
+        want = (position_side or "").strip().upper()
+        for row in rows:
+            amt = safe_float(row.get("positionAmt")) or 0.0
+            if abs(amt) < 1e-12:
+                continue
+            ps = str(row.get("positionSide") or "").upper()
+            if want and ps and ps not in ("BOTH",) and ps != want:
+                continue
+            return abs(float(amt))
+        return 0.0
+
+    def set_leverage(self, symbol: str, leverage: int | float) -> None:
+        lev = int(round(float(leverage)))
+        if lev < 1:
+            lev = 1
+        self._signed(
+            self._fapi,
+            "POST",
+            "/fapi/v1/leverage",
+            {"symbol": symbol, "leverage": lev},
+        )
+
     def get_funding_usdt(
         self, symbol: str, *, begin_ms: int, end_ms: int | None = None
     ) -> float: