feat: add MCP_RATE_LIMIT env variable to control MCP request rate

Document MCP_RATE_LIMIT in README, docker-compose, .env.example, Helm values and configmap.

docker-compose.yml

@@ -38,6 +38,7 @@ services:
 #      - OIDC_DISCOVERY_URL= # Override the OIDC discovery endpoint for providers with non-standard paths (e.g. Authentik)
 #      - ADMIN_EMAIL=admin@trek.local # Initial admin e-mail — only used on first boot when no users exist
 #      - ADMIN_PASSWORD=changeme      # Initial admin password — only used on first boot when no users exist
+#      - MCP_RATE_LIMIT=60 # Max MCP API requests per user per minute (default: 60)
     volumes:
       - ./data:/app/data
       - ./uploads:/app/uploads