feat(notify): Slack message for reconcile-booked closes (realized P&L)

Entries, deferrals and rejections posted to #trading-bot, but exits booked by the reconcile loop (bracket stop/take-profit legs that fill at Alpaca) were silent — the two Jun 9 stop-outs produced no message. Viktor asked for a Slack message on each position execution.

- move SlackNotifier to shared/ (now used by trade-executor AND api-gateway)

- add notify_close (ticker/qty/price/realized P&L/reason, win-loss emoji)

- reconcile loop notifies on each booked close, fail-soft; api-gateway config gains slack fields (channel defaults to trading-bot since the env carries only the bot token; chat:write.public covers posting)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/trade_executor/main.py

@@ -21,7 +21,7 @@ from sqlalchemy.ext.asyncio import async_sessionmaker
 from services.trade_executor.config import TradeExecutorConfig
 from services.trade_executor.deferred_queue import DeferredSignalQueue
 from services.trade_executor.risk_manager import RiskManager
-from services.trade_executor.slack_notifier import SlackNotifier
+from shared.slack_notifier import SlackNotifier
 from shared.broker.alpaca_broker import AlpacaBroker
 from shared.db import create_db
 from shared.models.trading import Trade as TradeModel