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/api/routes.py

@@ -103,6 +103,12 @@ def multi_status(runtime: GuardedRuntimeDep) -> MultiStatusOut:
     return runtime.multi_status()
 
 
+@router.get("/live-check")
+def live_check(runtime: GuardedRuntimeDep) -> dict:
+    """Preflight for live trading: whether the config is safe enough."""
+    return runtime.live_check()
+
+
 @router.post("/bot/start-many", response_model=MultiStatusOut)
 async def start_many(payload: StartManyRequest, runtime: GuardedRuntimeDep) -> MultiStatusOut:
     try: