feat: add point mall redemption

backend/src/main/java/com/openisle/service/PointMallService.java

@@ -0,0 +1,37 @@
+package com.openisle.service;
+
+import com.openisle.exception.FieldException;
+import com.openisle.exception.NotFoundException;
+import com.openisle.model.PointGood;
+import com.openisle.model.User;
+import com.openisle.repository.PointGoodRepository;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+/** Service for point mall operations. */
+@Service
+@RequiredArgsConstructor
+public class PointMallService {
+    private final PointGoodRepository pointGoodRepository;
+    private final UserRepository userRepository;
+    private final NotificationService notificationService;
+
+    public List<PointGood> listGoods() {
+        return pointGoodRepository.findAll();
+    }
+
+    public int redeem(User user, Long goodId, String contact) {
+        PointGood good = pointGoodRepository.findById(goodId)
+                .orElseThrow(() -> new NotFoundException("Good not found"));
+        if (user.getPoint() < good.getCost()) {
+            throw new FieldException("point", "Insufficient points");
+        }
+        user.setPoint(user.getPoint() - good.getCost());
+        userRepository.save(user);
+        notificationService.createActivityRedeemNotifications(user, good.getName() + ": " + contact);
+        return user.getPoint();
+    }
+}