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/runtime.py

@@ -11,6 +11,7 @@ from app.broker import build_broker
 from app.broker.base import Broker
 from app.config import Settings
 from app.engine.engine import TradingEngine
+from app.engine.live_guard import check_live_ready
 from app.engine.manager import MultiStrategyManager
 from app.engine.strategies import get_strategy
 from app.market import build_market_data
@@ -129,8 +130,17 @@ class Runtime:
         return engine.status() if engine is not None else self._idle_status()
 
     async def start_many(self, request: StartManyRequest) -> MultiStatusOut:
+        check = check_live_ready(
+            self.settings,
+            request.starting_balance or self.settings.starting_balance,
+        )
+        if not check.ok:
+            raise ValueError("live trading blocked: " + "; ".join(check.blockers))
         return await self.manager.start(request)
 
+    def live_check(self) -> dict:
+        return check_live_ready(self.settings).as_dict()
+
     async def stop(self) -> StatusOut:
         await self.manager.stop()
         engine = self.engine