feat: Add integration module with models, services, APIs and tests
- Create Integration module with agent_docs, datasource_configs, n8n_instances, agent_integrations tables
- Implement models, schemas and services for multi-tenant document agent integration
- Add n8n webhook endpoints (provider-status, sync-job-status)
- Add internal endpoints for sync jobs management
- Create comprehensive test suite with pytest
- Use user_uuid instead of user_id (external user microservice)
- Rename metadata to extra_metadata (SQLAlchemy reserved keyword)