feat: add RESET_TRAFFIC_ON_TARIFF_SWITCH admin setting

New boolean setting (default: True) controls whether user traffic
is reset when switching between tariff plans.

Changes:
- config.py: add RESET_TRAFFIC_ON_TARIFF_SWITCH setting
- system_settings_service.py: category override (TRAFFIC) + hints
- pricing.py: admin bot handler toggle entry
- cabinet/routes/subscription.py: pass reset_traffic to RemnaWave on switch
- webapi/routes/miniapp.py: same for miniapp tariff switch
- tariff_purchase.py: use setting in 3 switch handlers (was hardcoded)
- subscription_auto_purchase_service.py: separate tariff switch vs payment logic
- crud/subscription.py: conditional traffic_used_gb reset on tariff change

app/config.py

@@ -134,6 +134,7 @@ class Settings(BaseSettings):
     DEFAULT_DEVICE_LIMIT: int = 1
     DEFAULT_TRAFFIC_RESET_STRATEGY: str = 'MONTH'
     RESET_TRAFFIC_ON_PAYMENT: bool = False
+    RESET_TRAFFIC_ON_TARIFF_SWITCH: bool = True
     MAX_DEVICES_LIMIT: int = 20
 
     TRIAL_WARNING_HOURS: int = 2