feat: tariff switch direction control, fix device pricing within tariff limit

Tariff switch direction:
- Add TARIFF_SWITCH_UPGRADE_ENABLED and TARIFF_SWITCH_DOWNGRADE_ENABLED
  settings to control allowed switch directions
- Guard all 10 entry points: instant switch (list, preview, confirm),
  legacy switch (list, select, confirm, daily confirm), cabinet (preview,
  execute), purchase-options API
- Filter tariff lists by allowed direction, show "unavailable" when
  both directions disabled
- Expose settings in cabinet purchase-options response for frontend

Device pricing fix:
- Devices within tariff.device_limit are now free when restoring
  (was charging for all devices regardless of tariff inclusion)
- Fix max(100, price) minimum enforcing 1 RUB even when
  chargeable_devices is 0
- Apply fix across all endpoints: bot handlers (confirm_change,
  execute_change, confirm_add), cabinet API (legacy purchase,
  modern purchase, get-price, save-cart), inline keyboard display

app/keyboards/inline.py

@@ -2310,8 +2310,8 @@ def get_change_devices_keyboard(
     tariff_device_price = getattr(tariff, 'device_price_kopeks', None) if tariff else None
     if tariff and tariff_device_price:
         device_price_per_month = tariff_device_price
-        # Для тарифов все устройства платные (нет бесплатного лимита)
-        default_device_limit = 0
+        # Устройства в пределах тарифного лимита — бесплатные
+        default_device_limit = tariff.device_limit if tariff else 0
     else:
         device_price_per_month = settings.PRICE_PER_DEVICE
         default_device_limit = settings.DEFAULT_DEVICE_LIMIT