feat(api): add manual balance sync endpoint to z-dev

- Add POST /traders/:id/sync-balance endpoint
- Query actual exchange balance and update initial_balance
- Smart detection: shows balance change percentage
- Supports Binance, Hyperliquid, Aster exchanges
- Reload trader into memory after update

This complements the existing auto-query feature (feat/query-actual-balance-v2):
- Auto-query: Prevents issue at creation time
- Manual sync: Fixes issue after deposits/withdrawals

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

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

config/database.go

@@ -863,6 +863,12 @@ func (d *Database) UpdateTraderCustomPrompt(userID, id string, customPrompt stri
 	return err
 }
 
+// UpdateTraderInitialBalance 更新交易员初始余额（用于同步交易所实际余额）
+func (d *Database) UpdateTraderInitialBalance(userID, id string, newBalance float64) error {
+	_, err := d.db.Exec(`UPDATE traders SET initial_balance = ? WHERE id = ? AND user_id = ?`, newBalance, id, userID)
+	return err
+}
+
 // DeleteTrader 删除交易员
 func (d *Database) DeleteTrader(userID, id string) error {
 	_, err := d.db.Exec(`DELETE FROM traders WHERE id = ? AND user_id = ?`, id, userID)