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/config.py

@@ -39,6 +39,13 @@ class Settings(BaseSettings):
     exchange_api_secret: str = ""
     exchange_sandbox: bool = False
 
+    # --- Go-live gate ---
+    # Live trading only starts when this is exactly "I UNDERSTAND THE RISK".
+    # It is deliberately not a boolean: setting MODE=live alone is not enough.
+    live_confirmation: str = ""
+    # Refuse to start live below this account size (quote currency).
+    min_live_balance: float = 100.0
+
     # --- Risk controls (0 disables a limit) ---
     # Absolute loss in quote currency since the start of the UTC day.
     max_daily_loss: float = 0.0