Add category module and require post category

src/main/java/com/openisle/model/Category.java

@@ -0,0 +1,20 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "categories")
+public class Category {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(nullable = false, unique = true)
+    private String name;
+}

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

@@ -30,6 +30,10 @@ public class Post {
     @JoinColumn(name = "author_id")
     private User author;
 
+    @ManyToOne(optional = false, fetch = FetchType.LAZY)
+    @JoinColumn(name = "category_id")
+    private Category category;
+
     @Column(nullable = false)
     private long views = 0;