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.

README.md

@@ -144,6 +144,22 @@ injects it on the handshake). If `API_TOKEN` is unset, `/api/ws` is as open as
 the rest of `/api`, which is fine for a trusted/private deployment but is the
 reason to set a token if the host is exposed.
 
+## Going live
+
+Setting `MODE=live` alone is **not** enough. `GET /api/live-check` (and the
+dashboard banner) validate a preflight before any live order can be placed; live
+trading is blocked unless *all* of these hold:
+
+- at least one risk limit is active (`MAX_DAILY_LOSS`, `MAX_DRAWDOWN_PCT` or
+  `MAX_TRADES_PER_DAY`);
+- `MAX_POSITION_PCT < 1.0` (a single position can never be the whole account);
+- `LIVE_CONFIRMATION` is exactly `I UNDERSTAND THE RISK`;
+- the starting balance is at least `MIN_LIVE_BALANCE`;
+- exchange API credentials are set.
+
+A blocked live start returns `400` with the reasons, and the dashboard shows a
+banner listing them. Practise the kill switch in paper mode first.
+
 ## Multi-strategy
 
 The **Strategies (multi)** panel runs several strategies at once, each with its
@@ -193,6 +209,7 @@ Bundled strategies: `sma_cross`, `ema_cross`, `macd_trend`,
 | `POST` | `/api/bot/start` | Start a strategy |
 | `POST` | `/api/bot/start-many` | Start several strategies with per-strategy allocations |
 | `GET` | `/api/bot/status` | Per-strategy status for a multi-strategy run |
+| `GET` | `/api/live-check` | Go-live preflight (blockers for live trading) |
 | `POST` | `/api/bot/stop` | Stop the bot |
 | `POST` | `/api/bot/kill` | Kill switch: flatten + halt |
 | `POST` | `/api/backtest` | Backtest a strategy on historical candles |