refill hearts like Duolingo: one every 5 hours, all of them after 24

Each heart used to take 24 hours, so a student who lost all five waited up to
five days. Now one heart returns every 5 hours, and 24 hours after the first
heart was lost they all come back at once (the owner's call). /api/user/hearts
reports both times (nextRefillAt, fullRefillAt); an existing timer longer than
5 hours is shortened the first time it is read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

content/views.py

@@ -426,7 +426,7 @@ class QuestionAnswerView(APIView):
                     hearts.apply_regen()
                     if not has_unlimited_hearts(user_id):
                         hearts.lose_heart()
-                    hearts.save(update_fields=['current_hearts', 'next_refill_at'])
+                    hearts.save(update_fields=UserHearts.REGEN_FIELDS)
                     hearts_remaining = hearts.current_hearts
                 else:
                     hearts = UserHearts.objects.filter(user_id=user_id).first()