fix(payment): fix Alipay/Wxpay direct provider type mapping and enable cross-provider load balancing

Two issues fixed:

1. Alipay.SupportedTypes() returned ["alipay_direct"] and Wxpay returned
   ["wxpay_direct"], but the frontend sends payment_type="alipay"/"wxpay".
   The registry lookup failed with "payment method (alipay) is not
   configured". Fix: return the base types ["alipay"]/["wxpay"].

2. When multiple providers support the same payment type (e.g. EasyPay
   and Alipay direct both handle "alipay"), only the last-registered
   provider's instances were reachable — the registry mapped one type to
   one provider key, and SelectInstance queried by that single key.

   Fix: bypass the registry in invokeProvider and let SelectInstance
   query across all providers when providerKey is empty. The selected
   instance's own ProviderKey (now included in InstanceSelection) is
   used to create the correct provider, enabling true cross-provider
   load balancing.

Closes #1592

backend/internal/payment/provider/alipay.go

@@ -76,7 +76,7 @@ func (a *Alipay) getClient() (*alipay.Client, error) {
 func (a *Alipay) Name() string        { return "Alipay" }
 func (a *Alipay) ProviderKey() string { return payment.TypeAlipay }
 func (a *Alipay) SupportedTypes() []payment.PaymentType {
-	return []payment.PaymentType{payment.TypeAlipayDirect}
+	return []payment.PaymentType{payment.TypeAlipay}
 }
 
 // CreatePayment creates an Alipay payment page URL.

backend/internal/payment/provider/wxpay.go

@@ -72,7 +72,7 @@ func NewWxpay(instanceID string, config map[string]string) (*Wxpay, error) {
 func (w *Wxpay) Name() string        { return "Wxpay" }
 func (w *Wxpay) ProviderKey() string { return payment.TypeWxpay }
 func (w *Wxpay) SupportedTypes() []payment.PaymentType {
-	return []payment.PaymentType{payment.TypeWxpayDirect}
+	return []payment.PaymentType{payment.TypeWxpay}
 }
 
 func formatPEM(key, keyType string) string {