feat: add Mattermost webhook URL to n8n environment

Added MATTERMOST_WEBHOOK_URL environment variable to n8n:
- Available as environment variable in workflow expressions
- Can be used in HTTP Request nodes to send notifications
- Enables easy integration with Mattermost from any workflow

Usage in n8n workflows:
- Use {{ $env.MATTERMOST_WEBHOOK_URL }} in expressions
- Send POST requests with JSON payload to notify Mattermost

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

n8n/compose.yaml

@@ -30,6 +30,8 @@ services:
       N8N_SMTP_PASS: ${EMAIL_SMTP_PASSWORD}
       N8N_SMTP_SENDER: ${EMAIL_FROM}
       N8N_SMTP_SSL: "true"
+      # Mattermost webhook for notifications
+      MATTERMOST_WEBHOOK_URL: ${MATTERMOST_WEBHOOK_URL:-}
     depends_on:
       - postgres
     networks: