feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/payment/provider/factory.go

@@ -0,0 +1,23 @@
+package provider
+
+import (
+	"fmt"
+
+	"github.com/Wei-Shaw/sub2api/internal/payment"
+)
+
+// CreateProvider creates a Provider from a provider key, instance ID and decrypted config.
+func CreateProvider(providerKey string, instanceID string, config map[string]string) (payment.Provider, error) {
+	switch providerKey {
+	case "easypay":
+		return NewEasyPay(instanceID, config)
+	case "alipay":
+		return NewAlipay(instanceID, config)
+	case "wxpay":
+		return NewWxpay(instanceID, config)
+	case "stripe":
+		return NewStripe(instanceID, config)
+	default:
+		return nil, fmt.Errorf("unknown provider key: %s", providerKey)
+	}
+}