feat: add poll post type

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

@@ -0,0 +1,41 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+import java.time.LocalDateTime;
+import java.util.*;
+
+@Entity
+@Table(name = "poll_posts")
+@Getter
+@Setter
+@NoArgsConstructor
+@PrimaryKeyJoinColumn(name = "post_id")
+public class PollPost extends Post {
+
+    @Column(nullable = false)
+    private String question;
+
+    @ElementCollection
+    @CollectionTable(name = "poll_post_options", joinColumns = @JoinColumn(name = "post_id"))
+    @Column(name = "option_text")
+    private List<String> options = new ArrayList<>();
+
+    @ElementCollection
+    @CollectionTable(name = "poll_post_votes", joinColumns = @JoinColumn(name = "post_id"))
+    @MapKeyColumn(name = "option_index")
+    @Column(name = "vote_count")
+    private Map<Integer, Integer> votes = new HashMap<>();
+
+    @ManyToMany
+    @JoinTable(name = "poll_participants",
+            joinColumns = @JoinColumn(name = "post_id"),
+            inverseJoinColumns = @JoinColumn(name = "user_id"))
+    private Set<User> participants = new HashSet<>();
+
+    @Column
+    private LocalDateTime endTime;
+}