Implement balance monitoring and notification for Selectel API integration

- Added support for balance tracking using `SELECTEL_API_TOKEN` in the configuration.
- Introduced new balance notification logic in the local watchdog, alerting users on balance changes based on defined thresholds.
- Updated documentation to include instructions for setting up balance notifications and the required environment variables.
- Enhanced the `ready` and `session` modules to initialize balance state and handle notifications during GPU operations.
- Refactored the CLI and related components to support the new balance monitoring features, ensuring a seamless user experience.

env.example

@@ -63,3 +63,8 @@ IDLE_GRACE_MINUTES=45
 # LOCAL_WATCHDOG_GRACE_MINUTES=10
 PULL_OUTPUT=false
 NOTIFY_READY=true
+# Статический API-ключ панели (X-Token) — только для баланса, не для OpenStack:
+# SELECTEL_API_TOKEN=
+# Уведомления о списании (local-watchdog tick), шаг с момента up:
+# BALANCE_NOTIFY_STEP_RUB=200
+# BALANCE_NOTIFY_LOW_RUB=0