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.

tests/test_balance.py

@@ -0,0 +1,56 @@
+from gpu_rent.balance import (
+    BalanceWatchState,
+    balance_rub_from_payload,
+    plan_balance_notices,
+    spend_steps,
+)
+
+
+def test_balance_kopecks():
+    payload = {
+        "data": {
+            "billings": [
+                {"balances_values_sum": 123456},  # 1234.56 ₽
+            ]
+        }
+    }
+    assert balance_rub_from_payload(payload) == 1234.56
+
+
+def test_spend_steps_200():
+    assert spend_steps(1000, 850, 200) == 0
+    assert spend_steps(1000, 800, 200) == 1
+    assert spend_steps(1000, 600, 200) == 2
+    assert spend_steps(1000, 1100, 200) == 0  # top-up direction
+
+
+def test_plan_notifies_each_step():
+    state = BalanceWatchState(baseline_rub=1000.0)
+    state, msgs = plan_balance_notices(state, 750.0, step_rub=200.0)
+    assert state.last_notified_step == 1
+    assert len(msgs) == 1
+    assert "200" in msgs[0]
+    state, msgs2 = plan_balance_notices(state, 550.0, step_rub=200.0)
+    assert state.last_notified_step == 2
+    assert len(msgs2) == 1
+    # no re-notify same step
+    state, msgs3 = plan_balance_notices(state, 520.0, step_rub=200.0)
+    assert msgs3 == []
+    assert state.last_notified_step == 2
+
+
+def test_plan_topup_resets():
+    state = BalanceWatchState(baseline_rub=500.0, last_notified_step=2)
+    state, msgs = plan_balance_notices(state, 900.0, step_rub=200.0)
+    assert state.baseline_rub == 900.0
+    assert state.last_notified_step == 0
+    assert any("пополнен" in m for m in msgs)
+
+
+def test_plan_low_once():
+    state = BalanceWatchState(baseline_rub=1000.0)
+    state, msgs = plan_balance_notices(state, 150.0, step_rub=200.0, low_rub=200.0)
+    assert state.low_notified
+    assert any("низкий" in m for m in msgs)
+    state, msgs2 = plan_balance_notices(state, 100.0, step_rub=200.0, low_rub=200.0)
+    assert not any("низкий" in m for m in msgs2)