feat: relocate remaining dtos

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

@@ -0,0 +1,16 @@
+package com.openisle.dto;
+
+import lombok.Data;
+
+import java.time.LocalDateTime;
+
+/** Lightweight post metadata used in user profile lists. */
+@Data
+public class PostMetaDto {
+    private Long id;
+    private String title;
+    private String snippet;
+    private LocalDateTime createdAt;
+    private String category;
+    private long views;
+}