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/internal/server/routes/payment.go

@@ -37,6 +37,7 @@ func RegisterPaymentRoutes(
 			orders.GET("/:id", paymentHandler.GetOrder)
 			orders.POST("/:id/cancel", paymentHandler.CancelOrder)
 			orders.POST("/:id/refund-request", paymentHandler.RequestRefund)
+			orders.GET("/refund-eligible-providers", paymentHandler.GetRefundEligibleProviders)
 		}
 	}