feat: decouple cart from shared_lib, add app-owned models

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>

models/__init__.py

@@ -0,0 +1,2 @@
+from .order import Order, OrderItem
+from .page_config import PageConfig