feat(kevin_bridge): main orchestrator with dependency injection

Composable: cursor/aggregator/strategy/publisher/audit_writer/broker
all injected. Master kill-switch (kevin_enable_trading=false) routes to
audit-only path. Cursor advances ONLY after XADD succeeds (race fix).
Concrete collaborators wired in subsequent tasks.

Also extends TradeSignal + SignalDirection.EXIT with the optional
fields Kevin paths need (strategy_id, target_dollars, stop_loss_pct,
take_profit_pct).

shared/schemas/trading.py

@@ -1,6 +1,7 @@
 """Trading-related Pydantic schemas for Redis Streams messages and API payloads."""
 
-from datetime import datetime
+from datetime import UTC, datetime
+from decimal import Decimal
 from enum import Enum
 from typing import Any
 from uuid import UUID, uuid4
@@ -30,6 +31,7 @@ class SignalDirection(str, Enum):
     LONG = "LONG"
     SHORT = "SHORT"
     NEUTRAL = "NEUTRAL"
+    EXIT = "EXIT"
 
 
 # ---------------------------------------------------------------------------
@@ -102,7 +104,13 @@ class TradeSignal(BaseModel):
     strength: float = Field(ge=0.0, le=1.0)
     strategy_sources: list[str]
     sentiment_context: dict[str, Any] | None = None
-    timestamp: datetime
+    timestamp: datetime = Field(default_factory=lambda: datetime.now(UTC))
+
+    # --- Kevin v2 extensions (optional) ---
+    strategy_id: UUID | None = None
+    target_dollars: Decimal | None = None
+    stop_loss_pct: Decimal | None = None
+    take_profit_pct: Decimal | None = None
 
     model_config = {"from_attributes": True}