fix: keep block scanner awake while orders await confirmation

[zshuang15]

Problem

On EVM chains an order can get stuck in OrderStatusConfirming forever and never fire its notify — the payment is on-chain and has far more than enough confirmations, but the merchant is never credited. It happens when block_offset_confirm is enabled and the confirming order is the only pending order (typical for low-traffic merchants).

Root cause

Block scanning is demand-driven. syncBreak() halts the forward scan once hasLookbackOrders() finds no "receivable" orders, and receivableOrderStatuses() only returned {Waiting, Expired}:

func receivableOrderStatuses() []int {
	return []int{model.OrderStatusWaiting, model.OrderStatusExpired}
}

When an order transitions Waiting → Confirming and it is the only pending order, hasLookbackOrders() becomes false, the forward scan stops, and the in-memory chainBlockNum freezes.

With block_offset_confirm enabled, tradeConfirmHandle() gates confirmation on chainBlockNum - RefBlockNum >= ConfirmedOffset (e.g. 40 on Polygon). A frozen chainBlockNum therefore never reaches the threshold, so the order stays Confirming indefinitely — no notify, merchant never credited. It does not recover across restarts, because with no waiting/expired order the scanner correctly stays idle.

(With block_offset_confirm disabled the confirmation gate is skipped, so this only bites deployments that enable confirmations on an EVM chain — which is exactly the safe choice against reorgs.)

Fix

Include OrderStatusConfirming in receivableOrderStatuses() so the scanner keeps advancing while any order is awaiting confirmation.

Validation

Reproduced on Polygon (block_offset_confirm=1, ConfirmedOffset=40): a lone confirming order backed by a real, on-chain-confirmed USDT transfer (500+ confirmations) stayed in Confirming forever on an unpatched build — the scanner was idle and chainBlockNum was frozen. With this change the scanner stays awake, chainBlockNum advances, tradeConfirmHandle() promotes the order to success, and the notify fires.

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