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/app/api/routes.py

@@ -18,8 +18,10 @@ from app.schemas import (
     EventOut,
     KillRequest,
     ManualOrderRequest,
+    MultiStatusOut,
     OrderOut,
     PositionOut,
+    StartManyRequest,
     StartRequest,
     StatusOut,
     StrategyConfigIn,
@@ -96,6 +98,21 @@ async def start_bot(payload: StartRequest, runtime: GuardedRuntimeDep) -> Status
         raise HTTPException(status_code=400, detail=str(exc)) from exc
 
 
+@router.get("/bot/status", response_model=MultiStatusOut)
+def multi_status(runtime: GuardedRuntimeDep) -> MultiStatusOut:
+    return runtime.multi_status()
+
+
+@router.post("/bot/start-many", response_model=MultiStatusOut)
+async def start_many(payload: StartManyRequest, runtime: GuardedRuntimeDep) -> MultiStatusOut:
+    try:
+        return await runtime.start_many(payload)
+    except KeyError as exc:
+        raise HTTPException(status_code=404, detail=str(exc)) from exc
+    except (ValueError, RuntimeError) as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+
+
 @router.post("/bot/stop", response_model=StatusOut)
 async def stop_bot(runtime: GuardedRuntimeDep) -> StatusOut:
     return await runtime.stop()