fix: tariff switch pricing showing free for upgrades, admin duplicate subscription guard

- pricing_engine: use shortest period for daily rate comparison instead
  of period closest to remaining_days — fixes incorrect free/zero cost
  for upgrades when tariffs have different period sets
- pricing_engine: remove unused target_days parameter from
  get_tariff_daily_rate_fraction
- admin_users: add duplicate subscription check before create,
  change_tariff and activate actions to prevent UniqueViolationError
  on uq_subscriptions_user_tariff_active constraint
- admin_users: add IntegrityError fallback on create as TOCTOU safety net

app/cabinet/routes/admin_users.py

@@ -1050,6 +1050,17 @@ async def update_user_subscription(
                 detail='User already has a subscription. Enable multi-tariff mode to add more.',
             )
 
+        # Проверка: нельзя создать вторую активную подписку с тем же тарифом
+        if is_multi_tariff and request.tariff_id:
+            from app.database.crud.subscription import get_subscription_by_user_and_tariff
+
+            existing = await get_subscription_by_user_and_tariff(db, user.id, request.tariff_id)
+            if existing:
+                raise HTTPException(
+                    status_code=status.HTTP_409_CONFLICT,
+                    detail='User already has an active subscription for this tariff. Extend it instead.',
+                )
+
         from app.database.crud.subscription import create_paid_subscription
 
         days = request.days or 30
@@ -1069,16 +1080,25 @@ async def update_user_subscription(
                 if tariff.allowed_squads:
                     connected_squads = tariff.allowed_squads
 
-        new_sub = await create_paid_subscription(
-            db=db,
-            user_id=user.id,
-            duration_days=days,
-            traffic_limit_gb=traffic_limit,
-            device_limit=device_limit,
-            is_trial=is_trial,
-            tariff_id=request.tariff_id,
-            connected_squads=connected_squads,
-        )
+        from sqlalchemy.exc import IntegrityError
+
+        try:
+            new_sub = await create_paid_subscription(
+                db=db,
+                user_id=user.id,
+                duration_days=days,
+                traffic_limit_gb=traffic_limit,
+                device_limit=device_limit,
+                is_trial=is_trial,
+                tariff_id=request.tariff_id,
+                connected_squads=connected_squads,
+            )
+        except IntegrityError:
+            await db.rollback()
+            raise HTTPException(
+                status_code=status.HTTP_409_CONFLICT,
+                detail='User already has an active subscription for this tariff. Extend it instead.',
+            )
 
         # Sync to Remnawave panel
         await _sync_subscription_to_panel(db, user, new_sub)
@@ -1191,6 +1211,18 @@ async def update_user_subscription(
                 detail='Tariff not found',
             )
 
+        # Проверка: нельзя сменить тариф, если у пользователя уже есть
+        # другая активная подписка с целевым тарифом
+        if is_multi_tariff and request.tariff_id != subscription.tariff_id:
+            from app.database.crud.subscription import get_subscription_by_user_and_tariff
+
+            existing = await get_subscription_by_user_and_tariff(db, user.id, request.tariff_id)
+            if existing and existing.id != subscription.id:
+                raise HTTPException(
+                    status_code=status.HTTP_409_CONFLICT,
+                    detail='User already has an active subscription for the target tariff',
+                )
+
         # Preserve extra purchased devices above the old tariff's base limit
         from app.database.crud.subscription import calc_device_limit_on_tariff_switch
 
@@ -1322,6 +1354,18 @@ async def update_user_subscription(
         )
 
     if request.action == 'activate':
+        # Проверка: нельзя активировать, если у пользователя уже есть
+        # другая активная подписка с тем же тарифом
+        if is_multi_tariff and subscription.tariff_id:
+            from app.database.crud.subscription import get_subscription_by_user_and_tariff
+
+            existing = await get_subscription_by_user_and_tariff(db, user.id, subscription.tariff_id)
+            if existing and existing.id != subscription.id:
+                raise HTTPException(
+                    status_code=status.HTTP_409_CONFLICT,
+                    detail='Cannot activate: user already has an active subscription for this tariff',
+                )
+
         subscription.status = SubscriptionStatus.ACTIVE.value
         if subscription.end_date and subscription.end_date <= datetime.now(UTC):
             # Extend by 30 days if expired

app/services/pricing_engine.py

@@ -202,16 +202,20 @@ class PricingEngine:
     # ------------------------------------------------------------------
 
     @staticmethod
-    def get_tariff_daily_rate_fraction(tariff: Tariff, target_days: int) -> tuple[int, int]:
+    def get_tariff_daily_rate_fraction(tariff: Tariff) -> tuple[int, int]:
         """Дневная ставка тарифа как (price, period_days) для целочисленных вычислений.
 
         Возвращает числитель и знаменатель дроби price/period_days,
         чтобы избежать float-ошибок в финансовых расчётах.
+
+        Всегда использует кратчайший доступный период тарифа, чтобы
+        гарантировать корректное сравнение дневных ставок при смене
+        тарифов с разными наборами периодов.
         """
         periods = tariff.get_available_periods()
         if not periods:
             return 0, 1
-        best_period = min(periods, key=lambda p: abs(p - target_days))
+        best_period = min(periods)
         price = tariff.get_price_for_period(best_period)
         if not price or best_period <= 0:
             return 0, 1
@@ -270,8 +274,8 @@ class PricingEngine:
         # raw_cost = (new_p/new_d - cur_p/cur_d) * remaining
         #          = (new_p * cur_d - cur_p * new_d) * remaining / (new_d * cur_d)
         # Floor division (//) округляет дробные копейки вниз — в пользу пользователя.
-        cur_price, cur_period = self.get_tariff_daily_rate_fraction(current_tariff, remaining_days)
-        new_price, new_period = self.get_tariff_daily_rate_fraction(new_tariff, remaining_days)
+        cur_price, cur_period = self.get_tariff_daily_rate_fraction(current_tariff)
+        new_price, new_period = self.get_tariff_daily_rate_fraction(new_tariff)
 
         numerator = (new_price * cur_period - cur_price * new_period) * remaining_days
         denominator = new_period * cur_period