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>

progress/views.py

@@ -166,13 +166,14 @@ class UserHeartsView(APIView):
         user = request.user
         hearts, _ = UserHearts.objects.get_or_create(user_id=user.id)
         if hearts.apply_regen():
-            hearts.save(update_fields=['current_hearts', 'next_refill_at'])
+            hearts.save(update_fields=UserHearts.REGEN_FIELDS)
 
         unlimited = has_unlimited_hearts(user.id)
         data = {
             'currentHearts': hearts.max_hearts if unlimited else hearts.current_hearts,
             'maxHearts': hearts.max_hearts,
             'nextRefillAt': None if unlimited else hearts.next_refill_at,
+            'fullRefillAt': None if unlimited else hearts.full_refill_at,
             'isUnlimited': unlimited,
         }
         return Response(UserHeartsResponseSerializer(data).data)