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/service/payment_service.go

@@ -271,11 +271,17 @@ func psSliceContains(sl []string, s string) bool {
 	return false
 }
 
+// Subscription validity period unit constants.
+const (
+	validityUnitWeek  = "week"
+	validityUnitMonth = "month"
+)
+
 func psComputeValidityDays(days int, unit string) int {
 	switch unit {
-	case "week":
+	case validityUnitWeek:
 		return days * 7
-	case "month":
+	case validityUnitMonth:
 		return days * 30
 	default:
 		return days