fix(trader): prevent division by zero in autoSyncBalanceIfNeeded

- Add check for oldBalance <= 0 before calculating changePercent
- Directly update to actualBalance if initialBalance is invalid
- Add warning logs for database nil scenarios
- Prevent panic when initial_balance is 0 in database

Applies same critical fix from feat/auto-balance-sync (3a5dea7)
to z-dev branch with adapted database type handling.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

trader/auto_trader.go

@@ -336,6 +336,26 @@ func (at *AutoTrader) autoSyncBalanceIfNeeded() {
 	}
 
 	oldBalance := at.initialBalance
+
+	// 防止除以零：如果初始余额无效，直接更新为实际余额
+	if oldBalance <= 0 {
+		log.Printf("⚠️ [%s] 初始余额无效 (%.2f)，直接更新为实际余额 %.2f USDT", at.name, oldBalance, actualBalance)
+		at.initialBalance = actualBalance
+
+		if at.database != nil {
+			if err := at.database.UpdateTraderInitialBalance(at.userID, at.id, actualBalance); err != nil {
+				log.Printf("❌ [%s] 更新数据库失败: %v", at.name, err)
+			} else {
+				log.Printf("✅ [%s] 已自动同步余额到数据库", at.name)
+			}
+		} else {
+			log.Printf("⚠️ [%s] 数据库引用为空，余额仅在内存中更新", at.name)
+		}
+
+		at.lastBalanceSyncTime = time.Now()
+		return
+	}
+
 	changePercent := ((actualBalance - oldBalance) / oldBalance) * 100
 
 	// 变化超过5%才更新
@@ -354,6 +374,8 @@ func (at *AutoTrader) autoSyncBalanceIfNeeded() {
 			} else {
 				log.Printf("✅ [%s] 已自动同步余额到数据库", at.name)
 			}
+		} else {
+			log.Printf("⚠️ [%s] 数据库引用为空，余额仅在内存中更新", at.name)
 		}
 	} else {
 		log.Printf("✓ [%s] 余额变化不大 (%.2f%%)，无需更新", at.name, changePercent)