feat(trade-executor): Slack notifications on trade + risk-rejection

SlackNotifier posts a short message to a Slack incoming webhook on:
  - trade-executor submits an order (filled or pending)
  - RiskManager rejects a signal (except outside_market_hours, which
    spams every poll when the bot tries to trade after-hours)

Key properties:
  - No-op when slack_webhook_url is empty (fail-soft default).
  - HTTP errors are swallowed — a Slack outage MUST NOT crash the
    consumer loop; the trade already happened on Alpaca.
  - Kevin-strategy signals tagged "Meet Kevin" in the message so I can
    tell which strategy fired.

Wiring:
  - TradeExecutorConfig.slack_webhook_url + TRADING_SLACK_WEBHOOK_URL
    env var, sourced from Vault secret/trading-bot/slack_webhook_url
    via existing ExternalSecret.
  - SlackNotifier passed to process_signal; both rejection + post-trade
    paths call it.

Tests: 7 new (no-op when disabled, post calls webhook with correct
text, Kevin strategy tag, swallows HTTP errors, suppresses noisy
rejections).

services/trade_executor/main.py

@@ -19,6 +19,7 @@ from sqlalchemy.ext.asyncio import async_sessionmaker
 
 from services.trade_executor.config import TradeExecutorConfig
 from services.trade_executor.risk_manager import RiskManager
+from services.trade_executor.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
@@ -45,6 +46,7 @@ async def process_signal(
     publisher: StreamPublisher,
     counters: dict,
     db_session_factory: async_sessionmaker | None = None,
+    slack_notifier: SlackNotifier | None = None,
 ) -> None:
     """Process a single trade signal: risk check, order, record, publish.
 
@@ -68,6 +70,8 @@ async def process_signal(
     if not approved:
         logger.info("Signal REJECTED for %s: %s", signal.ticker, reason)
         counters["rejections"].add(1, {"reason": reason.split(" ")[0]})
+        if slack_notifier is not None:
+            await slack_notifier.notify_rejection(signal, reason)
         return
 
     # --- Step 2: calculate position size ---
@@ -149,6 +153,10 @@ async def process_signal(
         result.status.value,
     )
 
+    # --- Step 8: notify slack (best-effort, fail-soft) ---
+    if slack_notifier is not None:
+        await slack_notifier.notify_trade(signal, result)
+
 
 async def run(config: TradeExecutorConfig | None = None) -> None:
     """Main service loop.
@@ -196,6 +204,11 @@ async def run(config: TradeExecutorConfig | None = None) -> None:
     # --- Risk manager ---
     risk_manager = RiskManager(config, broker, redis=redis)
 
+    # --- Slack notifier (no-op when slack_webhook_url is empty) ---
+    slack_notifier = SlackNotifier(webhook_url=config.slack_webhook_url)
+    if slack_notifier.enabled:
+        logger.info("Slack notifications enabled")
+
     # --- Database (for persisting trades) ---
     db_session_factory = None
     try:
@@ -219,7 +232,15 @@ async def run(config: TradeExecutorConfig | None = None) -> None:
                 break
             try:
                 signal_msg = TradeSignal.model_validate(data)
-                await process_signal(signal_msg, risk_manager, broker, publisher, counters, db_session_factory)
+                await process_signal(
+                    signal_msg,
+                    risk_manager,
+                    broker,
+                    publisher,
+                    counters,
+                    db_session_factory,
+                    slack_notifier,
+                )
             except Exception:
                 logger.exception("Error processing signal: %s", data)
     finally: