Fix post count for older threads with lazy deleted forum posts

core/src/main/java/se/su/dsv/scipro/forum/dataobjects/ForumThread.java

@@ -15,6 +15,7 @@ import jakarta.persistence.Table;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
+import java.util.function.Predicate;
 import se.su.dsv.scipro.system.LazyDeletableDomainObject;
 import se.su.dsv.scipro.system.User;
 
@@ -100,8 +101,8 @@ public class ForumThread extends LazyDeletableDomainObject {
         posts.add(post);
     }
 
-    public int getPostCount() {
-        return posts.size();
+    public long getPostCount() {
+        return posts.stream().filter(Predicate.not(ForumPost::isDeleted)).count();
     }
 
     public User getCreatedBy() {