fix: classic mode renewal resets device_limit to 1 via cart key mismatch

- Fix cart key mismatch: extend cart saved 'device_limit' but
  confirm_purchase read 'devices' key, falling back to DEFAULT=1.
  Now both keys are saved in both cart-save paths
- Fix confirm_purchase device resolution: use explicit is None checks
  instead of or-chain to avoid falsy-zero trap
- Fix return_to_saved_cart display: fall back to 'device_limit' and
  'traffic_limit_gb' keys when 'devices'/'traffic_gb' are absent
- Fix second cart-save path in _extend_existing_subscription with
  same dual-key pattern
- Fix RemnaWaveService import path in renewal service
- Add RESET_DEVICES_ON_RENEWAL setting: resets all connected devices
  (hwid) via RemnaWave API on each subscription renewal

.env.example

@@ -263,6 +263,9 @@ SALES_MODE=tariffs
 TARIFF_SWITCH_UPGRADE_ENABLED=true
 TARIFF_SWITCH_DOWNGRADE_ENABLED=true
 
+# Сброс привязанных устройств при продлении подписки (однократно при каждом продлении)
+RESET_DEVICES_ON_RENEWAL=false
+
 # ===== ТРИАЛ ПОДПИСКА =====
 TRIAL_DURATION_DAYS=3
 TRIAL_TRAFFIC_LIMIT_GB=10