[AB-xxx] refactoring to use a separate updated column for the auto closing

wrapping modmail thread actions into a separate transaction

abstracto-application/abstracto-modules/modmail/modmail-int/src/main/java/dev/sheldan/abstracto/modmail/model/database/ModMailThread.java

@@ -56,6 +56,9 @@ public class ModMailThread implements Serializable {
     @Column(name = "updated", insertable = false, updatable = false)
     private Instant updated;
 
+    @Column(name = "last_updated")
+    private Instant lastUpdated;
+
     @Column(name = "closed")
     private Instant closed;