return mascotMood on answer submission

The math-app frontend prototype drives a RobotMascot with a fixed set of
moods; this gives that animation an explicit backend signal. Complements
the existing Lesson.mascot_animation / LessonIntro.mascotAnimation, which
are unchanged.

serializers.py
  - AnswerResponseSerializer: new mascotMood field.
views.py
  - QuestionAnswerView: mascotMood = 'happy' for a correct answer else
    'encourage', on both the fresh response and the idempotent-replay
    response.
tests.py
  - QuestionAnswerMascotMoodTests: correct -> happy, wrong -> encourage,
    and the mood is preserved across an idempotent replay.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/views.py

@@ -199,6 +199,7 @@ class QuestionAnswerView(APIView):
                     'correctOptionId': str(correct_choice.id) if correct_choice else '',
                     'xpEarned': existing.xp_earned,
                     'heartsRemaining': hearts.current_hearts if hearts else 5,
+                    'mascotMood': 'happy' if existing.correct else 'encourage',
                 }
                 return Response(AnswerResponseSerializer(data).data)
 
@@ -245,6 +246,7 @@ class QuestionAnswerView(APIView):
             'correctOptionId': str(correct_choice.id) if correct_choice else '',
             'xpEarned': xp_earned,
             'heartsRemaining': hearts_remaining if hearts_remaining is not None else 5,
+            'mascotMood': 'happy' if choice.is_correct else 'encourage',
         }
         return Response(AnswerResponseSerializer(data).data)