refactor(model): 将 Post和Draft 实体的内容字段类型从 TEXT 改为 LONGTEXT

backend/src/main/java/com/openisle/model/Draft.java

@@ -22,7 +22,7 @@ public class Draft {
 
     private String title;
 
-    @Column(columnDefinition = "TEXT")
+    @Column(columnDefinition = "LONGTEXT")
     private String content;
 
     @ManyToOne(fetch = FetchType.LAZY, optional = false)

backend/src/main/java/com/openisle/model/Post.java

@@ -31,7 +31,7 @@ public class Post {
     @Column(nullable = false)
     private String title;
 
-    @Column(nullable = false, columnDefinition = "TEXT")
+    @Column(nullable = false, columnDefinition = "LONGTEXT")
     private String content;
 
     @CreationTimestamp