feat: add post-wizard integration connect and toggle endpoints with Google Meet node-level control

Add PUT /integrations/connect endpoint to save credentials and inject them into N8N workflow Add PUT /integrations/{integrationUid}/toggle endpoint to enable/disable integrations and update workflow nodes Add getWorkflow() and updateWorkflow() methods to CoreN8nClient Auto-link Google Meet when Google Calendar is connected (shared credential) Add node name-based override in processNodeCredentials for Google Meet nodes that use googleCalendarOAuth2Api Make injectCredentials() and processNodeCredentials() static and package-private for reuse by integration service Add AGENT_NOT_ACTIVE error code (ASSISTANT_023) for non-active agent validation