feat: track poll votes

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

@@ -0,0 +1,33 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/**
+ * Represents a single vote in a poll, capturing which user selected which option.
+ */
+@Entity
+@Table(name = "poll_participants")
+@Getter
+@Setter
+@NoArgsConstructor
+public class PollParticipant {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "post_id", nullable = false)
+    private PollPost post;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "user_id", nullable = false)
+    private User user;
+
+    @Column(name = "option_index", nullable = false)
+    private int optionIndex;
+}
+

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

@@ -30,11 +30,8 @@ public class PollPost extends Post {
     @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<>();
+    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
+    private Set<PollParticipant> participants = new HashSet<>();
 
     @Column
     private LocalDateTime endTime;