feat: relocate remaining dtos

backend/src/main/java/com/openisle/dto/NotificationDto.java

@@ -0,0 +1,23 @@
+package com.openisle.dto;
+
+import com.openisle.model.NotificationType;
+import com.openisle.model.ReactionType;
+import lombok.Data;
+
+import java.time.LocalDateTime;
+
+/** DTO representing a user notification. */
+@Data
+public class NotificationDto {
+    private Long id;
+    private NotificationType type;
+    private PostSummaryDto post;
+    private CommentDto comment;
+    private CommentDto parentComment;
+    private AuthorDto fromUser;
+    private ReactionType reactionType;
+    private String content;
+    private Boolean approved;
+    private boolean read;
+    private LocalDateTime createdAt;
+}