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/models.py

@@ -1,4 +1,9 @@
+from datetime import timedelta
+
 from django.db import models
+from django.utils import timezone
+
+HEART_REFILL_INTERVAL = timedelta(hours=24)
 
 
 class UserStats(models.Model):
@@ -54,3 +59,36 @@ class UserHearts(models.Model):
 
     def __str__(self):
         return f"hearts-user{self.user_id}"
+
+    def apply_regen(self):
+        """اگه از next_refill_at گذشته باشه، هارت(های) گذشته رو پر می‌کنه (چندتا هم
+        اگه کاربر چند بازه‌ی HEART_REFILL_INTERVAL غایب بوده باشه). True یعنی
+        state عوض شده و باید save بشه."""
+        if self.current_hearts >= self.max_hearts:
+            if self.next_refill_at is not None:
+                self.next_refill_at = None
+                return True
+            return False
+
+        now = timezone.now()
+        if self.next_refill_at is None:
+            self.next_refill_at = now + HEART_REFILL_INTERVAL
+            return True
+        if now < self.next_refill_at:
+            return False
+
+        elapsed_intervals = 1 + (now - self.next_refill_at) // HEART_REFILL_INTERVAL
+        gained = min(elapsed_intervals, self.max_hearts - self.current_hearts)
+        self.current_hearts += gained
+        if self.current_hearts >= self.max_hearts:
+            self.next_refill_at = None
+        else:
+            self.next_refill_at += elapsed_intervals * HEART_REFILL_INTERVAL
+        return True
+
+    def lose_heart(self):
+        """یک هارت کم می‌کنه؛ اگه تایمرِ refill از قبل در حال شمارش نیست، از الان شروعش می‌کنه."""
+        if self.current_hearts > 0:
+            self.current_hearts -= 1
+        if self.next_refill_at is None and self.current_hearts < self.max_hearts:
+            self.next_refill_at = timezone.now() + HEART_REFILL_INTERVAL