feat: add per-provider allow_user_refund control and align wildcard matching

allow_user_refund:
- Add allow_user_refund field to PaymentProviderInstance ent schema
- Migration 103: ALTER TABLE payment_provider_instances ADD COLUMN
- Cascade logic: disabling refund_enabled auto-disables allow_user_refund
- User refund validation: check provider instance allows user refund
- Admin refund validation: check provider instance allows admin refund
- Subscription refund: deduct days on refund, rollback on failure
- New endpoint: GET /payment/orders/refund-eligible-providers
- Frontend: ToggleSwitch in ProviderCard/Dialog, cascade in SettingsView

Wildcard matching:
- Change findPricingForModel from "longest prefix wins" to "config order
  priority (first match wins)", aligning with channel service behavior

frontend/src/i18n/locales/en.ts

@@ -4646,6 +4646,7 @@ export default {
         supportedTypes: 'Supported Payment Types',
         supportedTypesHint: 'Comma-separated, e.g. alipay,wxpay',
         refundEnabled: 'Allow Refund',
+        allowUserRefund: 'Allow User Refund',
       },
       balanceNotify: {
         title: 'Balance Low Notification',