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/app/engine/live_guard.py

@@ -0,0 +1,62 @@
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+from app.config import Settings
+
+# The exact phrase required in LIVE_CONFIRMATION before live trading starts.
+LIVE_CONFIRMATION_PHRASE = "I UNDERSTAND THE RISK"
+
+
+@dataclass(slots=True)
+class LiveCheck:
+    """Result of the go-live preflight."""
+
+    ok: bool
+    mode: str
+    # Each failure is a human-readable reason live trading is blocked.
+    blockers: list[str] = field(default_factory=list)
+
+    def as_dict(self) -> dict:
+        return {"ok": self.ok, "mode": self.mode, "blockers": self.blockers}
+
+
+def check_live_ready(settings: Settings, starting_balance: float | None = None) -> LiveCheck:
+    """Validate that the configuration is safe enough to trade real money.
+
+    Paper mode always passes. Live mode must satisfy every guard; the point is
+    that ``MODE=live`` alone is not sufficient to start live trading.
+    """
+    if settings.mode != "live":
+        return LiveCheck(ok=True, mode=settings.mode)
+
+    blockers: list[str] = []
+
+    has_limit = (
+        settings.max_daily_loss > 0
+        or settings.max_drawdown_pct > 0
+        or settings.max_trades_per_day > 0
+    )
+    if not has_limit:
+        blockers.append(
+            "no active risk limit: set MAX_DAILY_LOSS, MAX_DRAWDOWN_PCT or MAX_TRADES_PER_DAY"
+        )
+
+    if settings.max_position_pct >= 1.0:
+        blockers.append(
+            "MAX_POSITION_PCT must be below 1.0 so a single position cannot be the whole account"
+        )
+
+    if settings.live_confirmation != LIVE_CONFIRMATION_PHRASE:
+        blockers.append(f'LIVE_CONFIRMATION must be exactly "{LIVE_CONFIRMATION_PHRASE}"')
+
+    balance = starting_balance if starting_balance is not None else settings.starting_balance
+    if balance < settings.min_live_balance:
+        blockers.append(
+            f"starting balance {balance:g} is below MIN_LIVE_BALANCE {settings.min_live_balance:g}"
+        )
+
+    if not settings.exchange_api_key or not settings.exchange_api_secret:
+        blockers.append("EXCHANGE_API_KEY and EXCHANGE_API_SECRET are required for live")
+
+    return LiveCheck(ok=not blockers, mode=settings.mode, blockers=blockers)