fix sale and adoption bugs

backend/src/main/java/com/petshop/backend/controller/SaleController.java

@@ -40,7 +40,7 @@ public class SaleController {
     }
 
     @PostMapping
-    @PreAuthorize("hasRole('STAFF')")
+    @PreAuthorize("hasAnyRole('STAFF', 'ADMIN')")
     public ResponseEntity<SaleResponse> createSale(@Valid @RequestBody SaleRequest request) {
         return ResponseEntity.status(HttpStatus.CREATED).body(saleService.createSale(request));
     }

backend/src/main/java/com/petshop/backend/dto/adoption/AdoptionRequest.java

@@ -20,6 +20,7 @@ public class AdoptionRequest {
 
     private Long employeeId;
 
+    @NotNull(message = "Source store ID is required")
     private Long sourceStoreId;
 
     private String paymentMethod;

backend/src/main/java/com/petshop/backend/dto/sale/SaleItemRequest.java

@@ -9,6 +9,7 @@ public class SaleItemRequest {
     private Long prodId;
 
     @NotNull(message = "Quantity is required")
+    @Positive(message = "Quantity must be positive")
     private Integer quantity;
 
     public Long getProdId() {

backend/src/main/java/com/petshop/backend/service/SaleService.java

@@ -172,7 +172,7 @@ public class SaleService {
             BigDecimal refundTotal;
 
             if (originalSubtotal != null && originalSubtotal.compareTo(BigDecimal.ZERO) > 0) {
-                BigDecimal ratio = subtotalAmount.divide(originalSubtotal, 10, RoundingMode.HALF_UP);
+                BigDecimal ratio = subtotalAmount.abs().divide(originalSubtotal, 10, RoundingMode.HALF_UP);
                 refundTotal = originalSale.getTotalAmount().abs().multiply(ratio).negate().setScale(2, RoundingMode.HALF_UP);
                 if (originalSale.getLoyaltyDiscountAmount() != null) {
                     loyaltyDiscountRefunded = originalSale.getLoyaltyDiscountAmount().multiply(ratio).setScale(2, RoundingMode.HALF_UP);