Go-live gate: refuse unsafe live trading

Setting MODE=live alone no longer starts trading. check_live_ready requires an
active risk limit, MAX_POSITION_PCT < 1.0, LIVE_CONFIRMATION exactly
"I UNDERSTAND THE RISK", a minimum balance, and exchange credentials. Enforced
in Runtime.start_many (both start paths) and surfaced at GET /api/live-check;
the dashboard shows a banner and blocks the start buttons. Adds tests.

backend/tests/test_live_guard.py

@@ -0,0 +1,91 @@
+from __future__ import annotations
+
+import pytest
+
+from app.config import Settings
+from app.engine.live_guard import LIVE_CONFIRMATION_PHRASE, check_live_ready
+
+
+def live_settings(**overrides) -> Settings:
+    base = {
+        "mode": "live",
+        "max_daily_loss": 100.0,
+        "max_position_pct": 0.5,
+        "live_confirmation": LIVE_CONFIRMATION_PHRASE,
+        "exchange_api_key": "k",
+        "exchange_api_secret": "s",
+        "starting_balance": 1000.0,
+        "min_live_balance": 100.0,
+    }
+    base.update(overrides)
+    return Settings(**base)
+
+
+def test_paper_mode_always_ok() -> None:
+    check = check_live_ready(Settings(mode="paper", max_position_pct=1.0))
+    assert check.ok is True
+    assert check.blockers == []
+
+
+def test_live_ok_when_fully_configured() -> None:
+    check = check_live_ready(live_settings())
+    assert check.ok is True
+
+
+def test_live_blocked_without_any_risk_limit() -> None:
+    check = check_live_ready(
+        live_settings(max_daily_loss=0, max_drawdown_pct=0, max_trades_per_day=0)
+    )
+    assert check.ok is False
+    assert any("risk limit" in b for b in check.blockers)
+
+
+def test_live_blocked_when_position_cap_is_full_account() -> None:
+    check = check_live_ready(live_settings(max_position_pct=1.0))
+    assert check.ok is False
+    assert any("MAX_POSITION_PCT" in b for b in check.blockers)
+
+
+def test_live_blocked_without_confirmation_phrase() -> None:
+    check = check_live_ready(live_settings(live_confirmation=""))
+    assert check.ok is False
+    assert any("LIVE_CONFIRMATION" in b for b in check.blockers)
+
+
+def test_live_blocked_below_min_balance() -> None:
+    check = check_live_ready(live_settings(starting_balance=50.0))
+    assert check.ok is False
+    assert any("MIN_LIVE_BALANCE" in b for b in check.blockers)
+
+
+def test_live_blocked_without_credentials() -> None:
+    check = check_live_ready(live_settings(exchange_api_key="", exchange_api_secret=""))
+    assert check.ok is False
+    assert any("EXCHANGE_API_KEY" in b for b in check.blockers)
+
+
+async def test_start_many_refuses_live_without_confirmation(settings) -> None:
+    from app.runtime import Runtime
+    from app.schemas import StartManyRequest, StrategySpec
+
+    settings.mode = "live"
+    settings.exchange_api_key = "k"
+    settings.exchange_api_secret = "s"
+    settings.max_position_pct = 1.0  # unsafe
+    runtime = Runtime(settings)
+    with pytest.raises(ValueError, match="live trading blocked"):
+        await runtime.start_many(
+            StartManyRequest(strategies=[StrategySpec(strategy_key="sma_cross")])
+        )
+
+
+def test_live_check_endpoint(runtime) -> None:
+    from fastapi.testclient import TestClient
+
+    from app.main import create_app
+
+    app = create_app(runtime.settings, runtime)
+    with TestClient(app) as client:
+        body = client.get("/api/live-check").json()
+        assert body["ok"] is True  # paper mode
+        assert body["mode"] == "paper"