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/schemas.py

@@ -134,6 +134,18 @@ class StrategyConfigOut(BaseModel):
     created_at: datetime
 
 
+class StrategySpec(BaseModel):
+    """One strategy in a multi-strategy run."""
+
+    strategy_key: str
+    params: dict[str, Any] = Field(default_factory=dict)
+    symbol: str | None = None
+    timeframe: str | None = None
+    # Fraction of the account allocated to this strategy (relative weights are
+    # normalised across the run, so [1, 3] gives 25% / 75%).
+    weight: float = Field(default=1.0, gt=0)
+
+
 class StartRequest(BaseModel):
     strategy_key: str
     symbol: str | None = None
@@ -142,6 +154,44 @@ class StartRequest(BaseModel):
     starting_balance: float | None = None
 
 
+class StartManyRequest(BaseModel):
+    strategies: list[StrategySpec] = Field(min_length=1)
+    starting_balance: float | None = None
+
+
+class StrategyStatusOut(BaseModel):
+    """Per-strategy status inside a multi-strategy run."""
+
+    strategy_key: str
+    strategy_name: str | None
+    params: dict[str, Any] = Field(default_factory=dict)
+    symbol: str
+    timeframe: str
+    allocation: float
+    cash: float
+    equity: float
+    realized_pnl: float
+    unrealized_pnl: float
+    total_pnl: float
+    open_positions: int
+    halted: bool = False
+    halt_reason: str | None = None
+    trades_today: int = 0
+    error: str | None = None
+
+
+class MultiStatusOut(BaseModel):
+    running: bool
+    mode: str
+    exchange: str
+    price: float | None = None
+    starting_balance: float
+    equity: float
+    total_pnl: float
+    strategies: list[StrategyStatusOut] = Field(default_factory=list)
+    last_tick: datetime | None = None
+
+
 class ManualOrderRequest(BaseModel):
     side: Literal["buy", "sell"]
     qty: float | None = None