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.

tests/shared/broker/test_alpaca_bracket.py

@@ -0,0 +1,89 @@
+"""Unit tests for BRACKET order support in OrderRequest + AlpacaBroker.
+
+No network — exercises the request-building path only.
+"""
+
+from __future__ import annotations
+
+import pytest
+from alpaca.trading.enums import OrderClass as AlpacaOrderClass
+from alpaca.trading.requests import MarketOrderRequest
+from pydantic import ValidationError
+
+from shared.broker.alpaca_broker import AlpacaBroker
+from shared.schemas.trading import OrderRequest, OrderSide, OrderType
+
+
+def _broker() -> AlpacaBroker:
+    return AlpacaBroker(api_key="test", secret_key="test", paper=True)
+
+
+def test_order_request_defaults_to_simple_class():
+    o = OrderRequest(ticker="NVDA", side=OrderSide.BUY, qty=10)
+    assert o.order_class == "simple"
+    assert o.take_profit_price is None
+    assert o.stop_loss_price is None
+
+
+def test_order_request_bracket_requires_both_legs():
+    with pytest.raises(ValidationError, match="bracket orders require"):
+        OrderRequest(
+            ticker="NVDA",
+            side=OrderSide.BUY,
+            qty=10,
+            order_class="bracket",
+            take_profit_price=200.0,
+            # stop_loss_price missing
+        )
+
+    with pytest.raises(ValidationError, match="bracket orders require"):
+        OrderRequest(
+            ticker="NVDA",
+            side=OrderSide.BUY,
+            qty=10,
+            order_class="bracket",
+            stop_loss_price=150.0,
+            # take_profit_price missing
+        )
+
+
+def test_order_request_bracket_with_both_legs_validates():
+    o = OrderRequest(
+        ticker="NVDA",
+        side=OrderSide.BUY,
+        qty=10,
+        order_class="bracket",
+        take_profit_price=200.0,
+        stop_loss_price=150.0,
+    )
+    assert o.order_class == "bracket"
+    assert o.take_profit_price == 200.0
+    assert o.stop_loss_price == 150.0
+
+
+def test_build_order_request_simple_market():
+    o = OrderRequest(ticker="NVDA", side=OrderSide.BUY, qty=10)
+    req = _broker()._build_order_request(o)
+    assert isinstance(req, MarketOrderRequest)
+    assert req.symbol == "NVDA"
+    assert req.qty == 10
+    # default MarketOrderRequest has order_class=None or SIMPLE
+    assert getattr(req, "order_class", None) in (None, AlpacaOrderClass.SIMPLE)
+
+
+def test_build_order_request_bracket_attaches_legs():
+    o = OrderRequest(
+        ticker="NVDA",
+        side=OrderSide.BUY,
+        qty=10,
+        order_class="bracket",
+        take_profit_price=200.0,
+        stop_loss_price=150.0,
+    )
+    req = _broker()._build_order_request(o)
+    assert isinstance(req, MarketOrderRequest)
+    assert req.order_class == AlpacaOrderClass.BRACKET
+    assert req.take_profit is not None
+    assert float(req.take_profit.limit_price) == 200.0
+    assert req.stop_loss is not None
+    assert float(req.stop_loss.stop_price) == 150.0