feat(kevin_bridge): service entrypoint with concrete wiring

Wires the dependency-injected KevinBridge to concrete Redis cursor +
DB session factory + AlpacaBroker (or stub when creds missing). Includes
TradeSignalPublisher (Pydantic -> dict for the redis stream) and
SIGINT/SIGTERM graceful shutdown. Adds is_asset_tradable + get_latest_price
to AlpacaBroker so the bridge can query asset metadata.

shared/broker/alpaca_broker.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 import asyncio
 import logging
 from datetime import datetime, timezone
+from decimal import Decimal
 
 from alpaca.common.exceptions import APIError
 from alpaca.trading.client import TradingClient
@@ -238,3 +239,32 @@ class AlpacaBroker(BaseBroker):
             self._client.get_order_by_id, order_id
         )
         return self._order_to_result(alpaca_order)
+
+    async def is_asset_tradable(self, symbol: str) -> bool:
+        """Return True iff Alpaca lists *symbol* as tradable.
+
+        Conservative on failure (returns False) so the bridge skips the
+        mention instead of half-trading it.
+        """
+        try:
+            asset = await asyncio.to_thread(self._client.get_asset, symbol)
+            return bool(getattr(asset, "tradable", False))
+        except Exception:
+            return False
+
+    async def get_latest_price(self, symbol: str) -> Decimal:
+        """Last trade price for *symbol* as Decimal. Returns Decimal('0')
+        on lookup failure — callers should treat 0 as 'no price available'.
+        """
+        try:
+            # alpaca-py uses MarketDataClient for prices; the TradingClient
+            # does not expose latest_trade. Use the get_assets fallback
+            # (returns last_close) when present.
+            asset = await asyncio.to_thread(self._client.get_asset, symbol)
+            for attr in ("last_trade_price", "close", "last_close"):
+                v = getattr(asset, attr, None)
+                if v is not None:
+                    return Decimal(str(v))
+        except Exception:
+            pass
+        return Decimal("0")