feat: add poll vote repository

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

@@ -0,0 +1,28 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+@Entity
+@Table(name = "poll_votes", uniqueConstraints = @UniqueConstraint(columnNames = {"post_id", "user_id"}))
+@Getter
+@Setter
+@NoArgsConstructor
+public class PollVote {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "post_id")
+    private PollPost post;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @Column(name = "option_index", nullable = false)
+    private int optionIndex;
+}