refactor(payment): code standards fixes and regression repairs

Backend:
- Split payment_order.go (546→314 lines) into payment_order_lifecycle.go
- Replace magic strings with constants in factory, easypay, webhook handler
- Add rate limit/validity unit constants in payment_order_lifecycle, payment_service
- Fix critical regression: add PaymentEnabled to GetPublicSettings response
- Add missing migration 099_fix_migrated_purchase_menu_label_icon.sql

Frontend:
- Fix StripePopupView.vue: replace `as any` with typed interface, use extractApiErrorMessage
- Fix AdminOrderTable.vue: replace hardcoded column labels with i18n t() calls
- Fix SubscriptionsView.vue: replace hardcoded Today/Tomorrow with i18n
- Extract duplicate statusBadgeClass/canRefund/formatOrderDateTime to orderUtils.ts
- Add missing i18n keys: common.today, common.tomorrow, payment.orders.orderType/actions
- Remove dead PurchaseSubscriptionView.vue (replaced by PaymentView)

backend/internal/payment/provider/factory.go

@@ -9,13 +9,13 @@ import (
 // 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":
+	case payment.TypeEasyPay:
 		return NewEasyPay(instanceID, config)
-	case "alipay":
+	case payment.TypeAlipay:
 		return NewAlipay(instanceID, config)
-	case "wxpay":
+	case payment.TypeWxpay:
 		return NewWxpay(instanceID, config)
-	case "stripe":
+	case payment.TypeStripe:
 		return NewStripe(instanceID, config)
 	default:
 		return nil, fmt.Errorf("unknown provider key: %s", providerKey)