Fix target-exit mismatch: correct close reason and executable PnL gate.

Locked premium exits no longer label as fixed_usdt; mark/book optimism no longer triggers close into a realized loss.

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

backend/app/strategy/exits.py

@@ -110,16 +110,17 @@ def check_exits(
     locked_exit_target: float | None = None,
 ) -> ExitDecision:
     """净盈利（预估全平后）≥ 所选模式目标则全平。持仓锁定目标优先。"""
+    # 模式始终按设置解析（权利金倍数 vs 固定），勿因锁定目标就改成 fixed_usdt
+    _resolved_target, mode = resolve_exit_target(
+        exit_mode=exit_mode,
+        net_profit_target=net_profit_target,
+        premium_exit_multiple=premium_exit_multiple,
+        initial_premium=initial_premium,
+    )
     if locked_exit_target is not None and float(locked_exit_target) > 0:
         target = float(locked_exit_target)
-        mode = EXIT_MODE_FIXED
     else:
-        target, mode = resolve_exit_target(
-            exit_mode=exit_mode,
-            net_profit_target=net_profit_target,
-            premium_exit_multiple=premium_exit_multiple,
-            initial_premium=initial_premium,
-        )
+        target = float(_resolved_target)
     if target > 0 and net_pnl + 1e-9 >= target:
         reason = "premium_multiple" if mode == EXIT_MODE_PREMIUM else "fixed_usdt"
         return ExitDecision(True, reason, target)