feat(phase2): BRACKET orders + Kevin risk caps (Tasks 18, 19)

Task 18 — OrderRequest + AlpacaBroker BRACKET support:
- OrderRequest gains order_class ("simple" | "bracket"),
  take_profit_price, stop_loss_price + model_validator that requires
  both legs when order_class == "bracket".
- AlpacaBroker._build_order_request branches to a MarketOrderRequest
  with OrderClass.BRACKET + TakeProfitRequest + StopLossRequest legs,
  TimeInForce.GTC so the bracket survives day boundaries.

Task 19 — RiskManager Kevin caps + circuit-breaker:
- TradeExecutorConfig gains 4 fields: kevin_daily_trade_cap,
  kevin_daily_alloc_cap_usd, kevin_equity_drawdown_halt_pct,
  kevin_daily_loss_circuit_pct.
- check_risk() applies the caps only when
  signal.strategy_id == KEVIN_STRATEGY_UUID; non-Kevin signals pass
  through the existing path unchanged.
- 4 new checks in order: drawdown halt (sets permanent
  trading:paused), daily-loss circuit (setex 24h), daily trade-count
  cap, daily allocation cap (rolling today's $ + this trade's
  notional).
- Counter keys: kevin:daily_trades:YYYY-MM-DD,
  kevin:daily_alloc_usd:YYYY-MM-DD, kevin:daily_pnl_usd:YYYY-MM-DD,
  kevin:starting_equity_usd. All read-only here; bridge + executor
  write them.

Tests: 5 bracket + 9 kevin-caps + 28 regression-safe. Total 67 + 14
new = 81 passing (excluding -m integration). No DB needed.

shared/schemas/trading.py

@@ -3,10 +3,10 @@
 from datetime import UTC, datetime
 from decimal import Decimal
 from enum import Enum
-from typing import Any
+from typing import Any, Literal
 from uuid import UUID, uuid4
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 
 class OrderType(str, Enum):
@@ -49,8 +49,22 @@ class OrderRequest(BaseModel):
     limit_price: float | None = None
     stop_price: float | None = None
 
+    order_class: Literal["simple", "bracket"] = "simple"
+    take_profit_price: float | None = None
+    stop_loss_price: float | None = None
+
     model_config = {"from_attributes": True}
 
+    @model_validator(mode="after")
+    def _bracket_requires_legs(self) -> "OrderRequest":
+        if self.order_class == "bracket" and (
+            self.take_profit_price is None or self.stop_loss_price is None
+        ):
+            raise ValueError(
+                "bracket orders require take_profit_price + stop_loss_price"
+            )
+        return self
+
 
 class OrderResult(BaseModel):
     """Returned after order submission or status query."""