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)

frontend/src/views/user/SubscriptionsView.vue

@@ -313,10 +313,10 @@ function formatExpirationDate(expiresAt: string): string {
   const dateStr = formatDateOnly(expires)
 
   if (days === 0) {
-    return `${dateStr} (Today)`
+    return `${dateStr} (${t('common.today')})`
   }
   if (days === 1) {
-    return `${dateStr} (Tomorrow)`
+    return `${dateStr} (${t('common.tomorrow')})`
   }
 
   return t('userSubscriptions.daysRemaining', { days }) + ` (${dateStr})`