fix heart refill and grant unlimited hearts to active subscribers

next_refill_at was set on UserHearts but never read or recalculated,
so a heart lost to zero never came back. UserHearts.apply_regen() now
catches up missed 24h intervals on read/deduct, and lose_heart() starts
the timer only when it isn't already running.

Any user with an active (non-expired) subscription now skips heart
deduction entirely, regardless of which plan they're on — good enough
for now until per-plan perks are needed.

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

content/views.py

@@ -11,6 +11,7 @@ from rest_framework.throttling import ScopedRateThrottle
 from rest_framework.views import APIView
 
 from progress.models import UserHearts, UserStats
+from subscription.services import has_unlimited_hearts
 
 from .models import Choice, Grade, Lesson, NodeProgress, Question, UserAnswer, UserLessonCompletion
 from .serializers import (
@@ -308,9 +309,10 @@ class QuestionAnswerView(APIView):
                 if not choice.is_correct:
                     UserHearts.objects.get_or_create(user_id=user_id)
                     hearts = UserHearts.objects.select_for_update().get(user_id=user_id)
-                    if hearts.current_hearts > 0:
-                        hearts.current_hearts -= 1
-                        hearts.save(update_fields=['current_hearts'])
+                    hearts.apply_regen()
+                    if not has_unlimited_hearts(user_id):
+                        hearts.lose_heart()
+                    hearts.save(update_fields=['current_hearts', 'next_refill_at'])
                     hearts_remaining = hearts.current_hearts
                 else:
                     hearts = UserHearts.objects.filter(user_id=user_id).first()