fix(api): remove trader from memory map after deletion

## Problem
After deleting a trader, the competition API still returns the deleted trader because:
- Database deletion succeeds
- But TraderManager.traders map still contains the trader
- GetCompetitionData() iterates over tm.traders map (includes deleted ones)

## Solution
Add RemoveTrader() method to TraderManager:
- Stop the trader if running
- Remove from traders map
- Simplified version (no competitionCache, z-dev doesn't have it)

## Changes
- Add manager/trader_manager.go::RemoveTrader()
- Update api/server.go::handleDeleteTrader() to call RemoveTrader()

## Comparison with PR NoFxAiOS#622
- PR NoFxAiOS#622: Full version with competitionCache clearing
- Ours: Simplified for z-dev (no competitionCache yet)
- Both solve the same core problem
- Ours is compatible with current z-dev architecture

## Related
- Inspired by PR NoFxAiOS#622 @songzhihui
- Complements frontend fix (commit 685f248)
- Frontend: instant UI updates
- Backend: correct data source

🤖 Generated with Claude Code
Co-Authored-By: Claude <[email protected]>

Author: the-dev-z

api/server.go

@@ -616,13 +616,10 @@ func (s *Server) handleDeleteTrader(c *gin.Context) {
 		return
 	}
 
-	// 如果交易员正在运行，先停止它
-	if trader, err := s.traderManager.GetTrader(traderID); err == nil {
-		status := trader.GetStatus()
-		if isRunning, ok := status["is_running"].(bool); ok && isRunning {
-			trader.Stop()
-			log.Printf("⏹  已停止运行中的交易员: %s", traderID)
-		}
+	// 从 TraderManager 中移除交易员（会自动停止运行中的交易员）
+	if err := s.traderManager.RemoveTrader(traderID); err != nil {
+		log.Printf("⚠️ 从管理器中移除交易员失败: %v", err)
+		// 不影响返回结果，因为数据库删除已成功
 	}
 
 	log.Printf("✓ 交易员已删除: %s", traderID)

manager/trader_manager.go

@@ -437,6 +437,31 @@ func (tm *TraderManager) GetTraderIDs() []string {
 	return ids
 }
 
+// RemoveTrader 从管理器中移除交易员
+func (tm *TraderManager) RemoveTrader(traderID string) error {
+	tm.mu.Lock()
+	defer tm.mu.Unlock()
+
+	// 检查交易员是否存在
+	trader, exists := tm.traders[traderID]
+	if !exists {
+		return fmt.Errorf("交易员不存在: %s", traderID)
+	}
+
+	// 如果交易员正在运行，先停止它
+	status := trader.GetStatus()
+	if isRunning, ok := status["is_running"].(bool); ok && isRunning {
+		trader.Stop()
+		log.Printf("⏹  已停止运行中的交易员: %s", traderID)
+	}
+
+	// 从 map 中删除
+	delete(tm.traders, traderID)
+	log.Printf("✓ 已从管理器中移除交易员: %s", traderID)
+
+	return nil
+}
+
 // StartAll 启动所有trader
 func (tm *TraderManager) StartAll() {
 	tm.mu.RLock()