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/tests.py

@@ -490,3 +490,54 @@ class LessonCompleteGuestThrottleBypassTests(TestCase):
         self.assertEqual(UserLessonCompletion.objects.filter(user_id=guest2.id).count(), 0)
         self.assertFalse(UserStats.objects.filter(user_id=guest2.id).exists())
         self.assertFalse(NodeProgress.objects.filter(user_id=guest2.id, node=self.node).exists())
+
+
+class QuestionAnswerMascotMoodTests(TestCase):
+    """پاسخِ /answer باید mascotMood بده: درست → happy، غلط → encourage
+    (هم‌خوان با RobotMood در MathApp_frontend)."""
+
+    def setUp(self):
+        from django.core.cache import cache
+        cache.clear()
+        self.client = APIClient()
+        self.user = User.objects.create(device_id='device-mascot-1', is_guest=True, name='')
+        self.token = issue_access_token(self.user.id)
+        grade = Grade.objects.create(title='پایه اول', icon='i', color='#fff')
+        unit = Unit.objects.create(grade=grade, title='فصل ۱', order=1)
+        node = Node.objects.create(unit=unit, title='گره ۱', order=1)
+        lesson = Lesson.objects.create(node=node, title='درس ۱', intro_text='...')
+        self.question = Question.objects.create(lesson=lesson, type='multiple_choice', prompt='۱+۱؟')
+        self.correct = Choice.objects.create(question=self.question, value='۲', is_correct=True)
+        self.wrong = Choice.objects.create(question=self.question, value='۳', is_correct=False)
+
+    def _answer(self, choice_id):
+        return self.client.post(
+            f'/api/questions/{self.question.id}/answer',
+            {'selectedOptionId': str(choice_id)}, format='json',
+            HTTP_AUTHORIZATION=f'Bearer {self.token}',
+        )
+
+    def test_correct_answer_mood_happy(self):
+        r = self._answer(self.correct.id)
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(r.data['mascotMood'], 'happy')
+
+    def test_wrong_answer_mood_encourage(self):
+        r = self._answer(self.wrong.id)
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(r.data['mascotMood'], 'encourage')
+
+    def test_idempotent_replay_keeps_mood(self):
+        k = 'mascot-idem-1'
+        first = self.client.post(
+            f'/api/questions/{self.question.id}/answer',
+            {'selectedOptionId': str(self.wrong.id)}, format='json',
+            HTTP_AUTHORIZATION=f'Bearer {self.token}', HTTP_IDEMPOTENCY_KEY=k,
+        )
+        replay = self.client.post(
+            f'/api/questions/{self.question.id}/answer',
+            {'selectedOptionId': str(self.wrong.id)}, format='json',
+            HTTP_AUTHORIZATION=f'Bearer {self.token}', HTTP_IDEMPOTENCY_KEY=k,
+        )
+        self.assertEqual(first.data['mascotMood'], 'encourage')
+        self.assertEqual(replay.data['mascotMood'], 'encourage')