Fix null multiple field for poll posts

feat(poll): use BaseSwitch for multiple selection

backend/src/main/java/com/openisle/mapper/PostMapper.java

@@ -111,7 +111,7 @@ public class PostMapper {
                     .collect(Collectors.groupingBy(PollVote::getOptionIndex,
                             Collectors.mapping(v -> userMapper.toAuthorDto(v.getUser()), Collectors.toList())));
             p.setOptionParticipants(optionParticipants);
-            p.setMultiple(pp.isMultiple());
+            p.setMultiple(Boolean.TRUE.equals(pp.getMultiple()));
             dto.setPoll(p);
         }
     }

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

@@ -33,7 +33,7 @@ public class PollPost extends Post {
     private Set<User> participants = new HashSet<>();
 
     @Column
-    private boolean multiple = false;
+    private Boolean multiple = false;
 
     @Column
     private LocalDateTime endTime;