chore: update Docker configuration files with n8n API options

- Added n8n API configuration section to docker-compose.yml
- Updated .env.docker with n8n API environment variables
- Added helpful comments explaining the 16 additional management tools
- Configuration remains optional and commented out by default

This makes it easier for users to enable n8n management tools in Docker deployments.

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

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

.env.docker

@@ -20,4 +20,10 @@ NODE_DB_PATH=/app/data/nodes.db
 REBUILD_ON_START=false
 
 # Optional nginx mode
-USE_NGINX=false
+USE_NGINX=false
+
+# Optional n8n API configuration (enables 16 additional management tools)
+# N8N_API_URL=https://your-n8n-instance.com
+# N8N_API_KEY=your-api-key-here
+# N8N_API_TIMEOUT=30000
+# N8N_API_MAX_RETRIES=3