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

backend/ent/schema/payment_provider_instance.go

@@ -53,6 +53,8 @@ func (PaymentProviderInstance) Fields() []ent.Field {
 			Default(""),
 		field.Bool("refund_enabled").
 			Default(false),
+		field.Bool("allow_user_refund").
+			Default(false),
 		field.Time("created_at").
 			Immutable().
 			Default(time.Now).