fix: resolve 3 code review issues in allow_user_refund

1. PrepareRefund: block refund on provider instance lookup failure
   instead of silently skipping permission check (medium severity)

2. UpdateProviderInstance: allow enabling refund_enabled and
   allow_user_refund in the same request by checking req.RefundEnabled
   value before falling back to DB read

3. ExecuteRefund: only revoke subscription on ErrAdjustWouldExpire,
   abort on other errors (DB failure, not found) instead of
   unconditionally revoking

backend/internal/service/payment_config_providers.go

@@ -231,10 +231,18 @@ func (s *PaymentConfigService) UpdateProviderInstance(ctx context.Context, id in
 		}
 	}
 	if req.AllowUserRefund != nil {
-		// Only allow enabling when refund_enabled is true
+		// Only allow enabling when refund_enabled is (or will be) true
 		if *req.AllowUserRefund {
-			inst, err := s.entClient.PaymentProviderInstance.Get(ctx, id)
-			if err == nil && inst.RefundEnabled {
+			refundEnabled := false
+			if req.RefundEnabled != nil {
+				refundEnabled = *req.RefundEnabled
+			} else {
+				inst, err := s.entClient.PaymentProviderInstance.Get(ctx, id)
+				if err == nil {
+					refundEnabled = inst.RefundEnabled
+				}
+			}
+			if refundEnabled {
 				u.SetAllowUserRefund(true)
 			}
 		} else {

backend/internal/service/payment_refund.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
 	"math"
@@ -93,15 +94,16 @@ func (s *PaymentService) PrepareRefund(ctx context.Context, oid int64, amt float
 	// Check provider instance allows admin refund
 	inst, instErr := s.getOrderProviderInstance(ctx, o)
 	if instErr != nil {
-		slog.Warn("refund: provider instance not found", "orderID", oid, "error", instErr)
+		slog.Warn("refund: provider instance lookup failed", "orderID", oid, "error", instErr)
+		return nil, nil, infraerrors.InternalServer("PROVIDER_LOOKUP_FAILED", "failed to look up payment provider for this order")
 	}
-	if inst != nil && !inst.RefundEnabled {
-		return nil, nil, infraerrors.Forbidden("REFUND_DISABLED", "refund is not enabled for this provider")
-	}
-	if inst == nil && instErr == nil {
+	if inst == nil {
 		// Legacy order without provider_instance_id — block refund
 		return nil, nil, infraerrors.Forbidden("REFUND_DISABLED", "refund is not available for this order")
 	}
+	if !inst.RefundEnabled {
+		return nil, nil, infraerrors.Forbidden("REFUND_DISABLED", "refund is not enabled for this provider")
+	}
 	if math.IsNaN(amt) || math.IsInf(amt, 0) {
 		return nil, nil, infraerrors.BadRequest("INVALID_AMOUNT", "invalid refund amount")
 	}
@@ -183,10 +185,17 @@ func (s *PaymentService) ExecuteRefund(ctx context.Context, p *RefundPlan) (*Ref
 		if !s.hasAuditLog(ctx, p.OrderID, "REFUND_ROLLBACK_FAILED") {
 			_, err := s.subscriptionSvc.ExtendSubscription(ctx, p.SubscriptionID, -p.SubDaysToDeduct)
 			if err != nil {
-				slog.Info("subscription deduction would expire, revoking", "orderID", p.OrderID, "subID", p.SubscriptionID, "days", p.SubDaysToDeduct)
-				if revokeErr := s.subscriptionSvc.RevokeSubscription(ctx, p.SubscriptionID); revokeErr != nil {
+				if errors.Is(err, ErrAdjustWouldExpire) {
+					// Deduction would expire the subscription — revoke it entirely
+					slog.Info("subscription deduction would expire, revoking", "orderID", p.OrderID, "subID", p.SubscriptionID, "days", p.SubDaysToDeduct)
+					if revokeErr := s.subscriptionSvc.RevokeSubscription(ctx, p.SubscriptionID); revokeErr != nil {
+						s.restoreStatus(ctx, p)
+						return nil, fmt.Errorf("revoke subscription: %w", revokeErr)
+					}
+				} else {
+					// Other errors (DB failure, not found) — abort refund
 					s.restoreStatus(ctx, p)
-					return nil, fmt.Errorf("revoke subscription: %w", revokeErr)
+					return nil, fmt.Errorf("deduct subscription days: %w", err)
 				}
 			}
 		} else {