feat: add Discord webhook notifications to MTF dry-run bot

Sends alerts for: bot start, virtual entry (LONG/SHORT with SL/TP),
and SL/TP exits with PnL. Uses existing DiscordNotifier via
DISCORD_WEBHOOK_URL from .env. Also added env_file to mtf-bot
docker-compose service.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -56,6 +56,8 @@ services:
     image: git.gihyeon.com/gihyeon/cointrader:latest
     container_name: mtf-bot
     restart: unless-stopped
+    env_file:
+      - .env
     environment:
       - TZ=Asia/Seoul
       - PYTHONUNBUFFERED=1