Multi-strategy engine with per-strategy capital allocation

Adds MultiStrategyManager: it runs several strategies concurrently, each with
its own TradingEngine seeded with starting_balance * allocation, so one
strategy's losses cannot eat into another's capital. Weights are normalised
across the run ([1,3] -> 25%/75%). Risk limits stay per strategy (each engine
owns a RiskManager).

New API: POST /api/bot/start-many and GET /api/bot/status. The existing
single-strategy endpoints delegate to the manager, so their behaviour is
unchanged. Adds manager tests (allocation, isolation, duplicate keys) and API
tests.

backend/tests/test_api.py

@@ -111,6 +111,42 @@ def test_backtest_limit_validation(runtime: Runtime) -> None:
         )
 
 
+def test_start_many_and_multi_status(runtime: Runtime, market: FakeMarketData) -> None:
+    app = create_app(runtime.settings, runtime)
+    with TestClient(app) as client:
+        started = client.post(
+            "/api/bot/start-many",
+            json={
+                "strategies": [
+                    {"strategy_key": "sma_cross", "params": {"fast": 2, "slow": 4}, "weight": 1},
+                    {"strategy_key": "ema_cross", "params": {"fast": 2, "slow": 4}, "weight": 3},
+                ],
+                "starting_balance": 10000,
+            },
+        )
+        assert started.status_code == 200
+        body = started.json()
+        assert body["running"] is True
+        assert len(body["strategies"]) == 2
+        assert [s["allocation"] for s in body["strategies"]] == [0.25, 0.75]
+
+        status = client.get("/api/bot/status").json()
+        assert status["running"] is True
+        assert len(status["strategies"]) == 2
+        assert status["equity"] == 10000
+
+        client.post("/api/bot/stop")
+
+
+def test_start_many_rejects_unknown_strategy(runtime: Runtime) -> None:
+    app = create_app(runtime.settings, runtime)
+    with TestClient(app) as client:
+        response = client.post(
+            "/api/bot/start-many", json={"strategies": [{"strategy_key": "nope"}]}
+        )
+        assert response.status_code == 404
+
+
 def test_kill_endpoint(runtime: Runtime) -> None:
     app = create_app(runtime.settings, runtime)
     with TestClient(app) as client: