return flag on submitted successfully

service/src/main/java/com/telephoners/krakyournet/ctf/resources/SolutionsResource.java

@@ -52,7 +52,7 @@ public class SolutionsResource
                 .orElseThrow(InvalidSolutionException::new);
 
         solutionsRepository.submitSolution(new Solution(teamsRepository.getTeamByUser(user), task, flag));
-        return Response.ok().build();
+        return Response.ok().entity(flag).build();
     }
 
     @GET