feat: add LIMITED subscription status and preserve extra devices on tariff switch

- Add SubscriptionStatus.LIMITED for traffic-exhausted subscriptions
- Webhook user.limited now sets LIMITED directly instead of DISABLED
- Add LIMITED to reactivation, extend, resume, auto-purchase, contest eligibility
- Add traffic_exhausted error response in miniapp API
- Fix device_limit being overwritten on tariff switch in all code paths:
  admin change_tariff, user switch-tariff, miniapp, bot tariff_purchase,
  auto_purchase_service — now preserves extra purchased devices via
  calc_device_limit_on_tariff_switch() helper
- Fix truthiness checks on device_limit (0 is valid, use `is not None`)

app/database/models.py

@@ -123,6 +123,7 @@ class SubscriptionStatus(Enum):
     ACTIVE = 'active'
     EXPIRED = 'expired'
     DISABLED = 'disabled'
+    LIMITED = 'limited'
     PENDING = 'pending'
 
 
@@ -1350,6 +1351,9 @@ class Subscription(Base):
         if self.status == SubscriptionStatus.DISABLED.value:
             return 'disabled'
 
+        if self.status == SubscriptionStatus.LIMITED.value:
+            return 'limited'
+
         if self.status == SubscriptionStatus.ACTIVE.value:
             if end is None or end <= current_time:
                 return 'expired'
@@ -1374,6 +1378,8 @@ class Subscription(Base):
             return '🟢 Активна'
         if actual_status == 'disabled':
             return '⚫ Отключена'
+        if actual_status == 'limited':
+            return '⚠️ Трафик исчерпан'
         if actual_status == 'trial':
             return '🎯 Тестовая'
 
@@ -1391,6 +1397,8 @@ class Subscription(Base):
             return '💎'
         if actual_status == 'disabled':
             return '⚫'
+        if actual_status == 'limited':
+            return '⚠️'
         if actual_status == 'trial':
             return '🎁'
 
@@ -1439,7 +1447,7 @@ class Subscription(Base):
         else:
             self.end_date = datetime.now(UTC) + timedelta(days=days)
 
-        if self.status == SubscriptionStatus.EXPIRED.value:
+        if self.status in (SubscriptionStatus.EXPIRED.value, SubscriptionStatus.LIMITED.value):
             self.status = SubscriptionStatus.ACTIVE.value
 
     def add_traffic(self, gb: int):