fix compatibility regressions

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

@@ -4,7 +4,6 @@ import com.petshop.backend.dto.ai.AiChatRequest;
 import com.petshop.backend.dto.ai.AiChatResponse;
 import com.petshop.backend.entity.Pet;
 import com.petshop.backend.entity.User;
-import com.petshop.backend.exception.BusinessException;
 import com.petshop.backend.repository.PetRepository;
 import com.petshop.backend.service.OpenRouterService;
 import com.petshop.backend.util.AuthenticationHelper;
@@ -37,7 +36,7 @@ public class AiChatController {
     @PreAuthorize("isAuthenticated()")
     public ResponseEntity<AiChatResponse> sendMessage(@Valid @RequestBody AiChatRequest request) {
         if (request.getMessage() == null || request.getMessage().isBlank()) {
-            throw new BusinessException("Message cannot be empty");
+            return ResponseEntity.badRequest().body(AiChatResponse.fail("Message cannot be empty"));
         }
         ContentFilter.validate(request.getMessage());
 

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

@@ -279,22 +279,17 @@ public class AuthController {
     }
 
     @PostMapping("/me/avatar")
-    public ResponseEntity<AvatarUploadResponse> uploadAvatar(@RequestParam("avatar") MultipartFile file) {
+    public ResponseEntity<AvatarUploadResponse> uploadAvatar(@RequestParam("avatar") MultipartFile file) throws IOException {
         User user = authHelper.getAuthenticatedUser();
 
         ImageValidationUtil.validate(file);
 
-        try {
         avatarStorageService.deleteAvatar(user);
         String avatarPath = avatarStorageService.storeAvatar(file);
         user.setAvatarUrl(avatarPath);
         userRepository.save(user);
 
         return ResponseEntity.ok(new AvatarUploadResponse(avatarStorageService.toOwnerAvatarUrl(user), "Avatar uploaded successfully"));
-
-        } catch (IOException e) {
-            throw new BusinessException("Failed to upload avatar: " + e.getMessage());
-        }
     }
 
     @GetMapping("/me/avatar")

backend/src/main/java/com/petshop/backend/exception/GlobalExceptionHandler.java

@@ -2,6 +2,7 @@ package com.petshop.backend.exception;
 
 import com.petshop.backend.service.PetService;
 import jakarta.servlet.http.HttpServletRequest;
+import java.io.IOException;
 import org.springframework.dao.DataIntegrityViolationException;
 import org.springframework.data.core.PropertyReferenceException;
 import org.springframework.http.HttpStatus;
@@ -121,6 +122,11 @@ public class GlobalExceptionHandler {
         return buildErrorResponse(HttpStatus.FORBIDDEN, "Access to this pet image is not allowed", ex, request);
     }
 
+    @ExceptionHandler(IOException.class)
+    public ResponseEntity<ApiErrorResponse> handleIOException(IOException ex, HttpServletRequest request) {
+        return buildErrorResponse(HttpStatus.BAD_REQUEST, ex.getMessage(), ex, request);
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<ApiErrorResponse> handleGenericException(Exception ex, HttpServletRequest request) {
         String message = ex.getMessage() == null || ex.getMessage().isBlank()

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

@@ -139,11 +139,15 @@ public class CouponService {
     }
 
     public static boolean isPercentageType(String discountType) {
-        return "PERCENTAGE".equalsIgnoreCase(discountType) || "PERCENT".equalsIgnoreCase(discountType);
+        if (discountType == null) return false;
+        String type = discountType.trim();
+        return "PERCENTAGE".equalsIgnoreCase(type) || "PERCENT".equalsIgnoreCase(type);
     }
 
     public static boolean isFixedType(String discountType) {
-        return "FIXED".equalsIgnoreCase(discountType) || "FLAT".equalsIgnoreCase(discountType);
+        if (discountType == null) return false;
+        String type = discountType.trim();
+        return "FIXED".equalsIgnoreCase(type) || "FLAT".equalsIgnoreCase(type);
     }
 
     private CouponResponse mapToResponse(Coupon coupon) {