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.
