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/types/payment.ts

@@ -89,6 +89,7 @@ export interface PaymentOrder {
   refund_requested_by?: number
   refund_request_reason?: string
   plan_id?: number
+  provider_instance_id?: string
 }
 
 // ==================== Plans & Channels ====================
@@ -138,6 +139,7 @@ export interface ProviderInstance {
   enabled: boolean
   payment_mode: string
   refund_enabled: boolean
+  allow_user_refund: boolean
   limits: string
   sort_order: number
 }