fix: 重启后加仓数据恢复 + scale_count/amount_usd 实时持久化

- restoreOpenPositions 从 orders 表 count 加仓次数，正确设置 ScaleLevels 和 AmountUSD
- 加仓时立即持久化 amount_usd 和 scale_count 到 trades 表（不等到平仓）
- PnL 图表支持单笔交易显示（原点+累计PnL连线）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

db/trade_repo.go

@@ -118,6 +118,12 @@ func (d *DB) UpdateTradeEntry(id int64, t *TradeRecord) error {
 	return err
 }
 
+// UpdateTradeScale updates scale-in fields on an existing trade (amount_usd, scale_count).
+func (d *DB) UpdateTradeScale(id int64, amountUSD float64, scaleCount int) error {
+	_, err := d.Exec("UPDATE trades SET amount_usd=?, scale_count=? WHERE id=?", amountUSD, scaleCount, id)
+	return err
+}
+
 // GetOpenTrades returns all non-closed trades (status='open' or status='entering').
 func (d *DB) GetOpenTrades() ([]TradeRecord, error) {
 	rows, err := d.Query(`SELECT id, coin, direction, status, entry_spread, exit_spread,