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/tests/test_exit_lock.py

@@ -5,6 +5,21 @@ from __future__ import annotations
 from app.strategy.exits import check_exits, resolve_exit_target
 
 
+def test_locked_premium_mode_keeps_premium_reason() -> None:
+    """锁定目标金额后，平仓原因仍应按权利金倍数模式标记，而非写死 fixed_usdt。"""
+    d = check_exits(
+        net_pnl=25.0,
+        exit_mode="premium_multiple",
+        net_profit_target=15.0,
+        premium_exit_multiple=1.0,
+        initial_premium=20.0,
+        locked_exit_target=20.0,
+    )
+    assert d.should_close is True
+    assert d.target == 20.0
+    assert d.reason == "premium_multiple"
+
+
 def test_locked_target_ignores_setting_drift() -> None:
     d = check_exits(
         net_pnl=20.0,