feat: add notification system

src/main/java/com/openisle/service/NotificationService.java

@@ -0,0 +1,48 @@
+package com.openisle.service;
+
+import com.openisle.model.*;
+import com.openisle.repository.NotificationRepository;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+/** Service for creating and retrieving notifications. */
+@Service
+@RequiredArgsConstructor
+public class NotificationService {
+    private final NotificationRepository notificationRepository;
+    private final UserRepository userRepository;
+
+    public Notification createNotification(User user, NotificationType type, Post post, Comment comment, Boolean approved) {
+        Notification n = new Notification();
+        n.setUser(user);
+        n.setType(type);
+        n.setPost(post);
+        n.setComment(comment);
+        n.setApproved(approved);
+        return notificationRepository.save(n);
+    }
+
+    public List<Notification> listNotifications(String username, Boolean read) {
+        User user = userRepository.findByUsername(username)
+                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        if (read == null) {
+            return notificationRepository.findByUserOrderByCreatedAtDesc(user);
+        }
+        return notificationRepository.findByUserAndReadOrderByCreatedAtDesc(user, read);
+    }
+
+    public void markRead(String username, List<Long> ids) {
+        User user = userRepository.findByUsername(username)
+                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        List<Notification> notifs = notificationRepository.findAllById(ids);
+        for (Notification n : notifs) {
+            if (n.getUser().getId().equals(user.getId())) {
+                n.setRead(true);
+            }
+        }
+        notificationRepository.saveAll(notifs);
+    }
+}