Phase 1-3 of decoupling:
- path_setup.py adds project root to sys.path
- Cart-owned models in cart/models/ (order, page_config)
- All imports updated: shared.infrastructure, shared.db, shared.browser, etc.
- PageConfig uses container_type/container_id instead of post_id FK
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>