Add reaction module

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

@@ -0,0 +1,40 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/**
+ * Reaction entity representing a user's reaction to a post or comment.
+ */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "reactions",
+       uniqueConstraints = {
+           @UniqueConstraint(columnNames = {"user_id", "post_id"}),
+           @UniqueConstraint(columnNames = {"user_id", "comment_id"})
+       })
+public class Reaction {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Enumerated(EnumType.STRING)
+    @Column(nullable = false)
+    private ReactionType type;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "post_id")
+    private Post post;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "comment_id")
+    private Comment comment;
+}

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

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+package com.openisle.model;
+
+/**
+ * Enum of possible reaction types for posts and comments.
+ */
+public enum ReactionType {
+    LIKE,
+    DISLIKE,
+    RECOMMEND,
+    ANGRY
+}