fix: bind trade callback event loop

[doskoi]

Summary

• bind BridgeTraderCallback to the running FastAPI asyncio loop after the trading manager connects
• keep the change limited to trading startup
• add a lifespan test that verifies callbacks receive the running loop

Why

BridgeTraderCallback dispatches xttrader background-thread events with asyncio.run_coroutine_threadsafe. Without setting the FastAPI loop during startup, trade websocket clients and notifiers may not receive order/trade/error callbacks.

Tests

• uv run --with pytest --with fastapi python -m pytest tests/test_trade_callback_event_loop.py -q