fix(api): add balance sync endpoint with smart detection

## Summary
- Add POST /traders/:id/sync-balance endpoint (Option B)
- Add smart detection showing balance change percentage (Option C)
- Fix balance display bug caused by commit 2b9c4d2
## Changes
### api/server.go
- Add handleSyncBalance() handler
- Query actual exchange balance via trader.GetBalance()
- Calculate change percentage for smart detection
- Update initial_balance in database
- Reload trader into memory after update
### config/database.go
- Add UpdateTraderInitialBalance() method
- Update traders.initial_balance field
## Root Cause
Commit 2b9c4d2 auto-queries exchange balance at trader creation time,
but never updates after user deposits more funds, causing:
- Wrong initial_balance (400 USDT vs actual 3000 USDT)
- Wrong P&L calculations (-2598.55 USDT instead of actual)
## Solution
Provides manual sync API + smart detection to update initial_balance
when user deposits funds after trader creation.
Co-Authored-By: tinkle-community <tinklefund@gmail.com>

config/database.go

@@ -853,6 +853,12 @@ 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)