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>

progress/serializers.py

@@ -39,3 +39,4 @@ class UserHeartsResponseSerializer(serializers.Serializer):
     currentHearts = serializers.IntegerField()
     maxHearts = serializers.IntegerField()
     nextRefillAt = serializers.DateTimeField(allow_null=True)
+    isUnlimited = serializers.BooleanField()