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/handler/payment_handler.go

@@ -335,6 +335,16 @@ func (h *PaymentHandler) RequestRefund(c *gin.Context) {
 	response.Success(c, gin.H{"message": "refund requested"})
 }
 
+// GetRefundEligibleProviders returns provider instance IDs that allow user refund.
+func (h *PaymentHandler) GetRefundEligibleProviders(c *gin.Context) {
+	ids, err := h.configService.GetUserRefundEligibleInstanceIDs(c.Request.Context())
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	response.Success(c, gin.H{"provider_instance_ids": ids})
+}
+
 // VerifyOrderRequest is the request body for verifying a payment order.
 type VerifyOrderRequest struct {
 	OutTradeNo string `json:"out_trade_no" binding:"required"`