feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infrastructure/cart_loader.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from quart import g
+
+
+async def load_cart():
+    # Lazy import: cart.bp.cart.services only exists in the cart app process.
+    # This avoids cross-app model conflicts at import time.
+    from cart.bp.cart.services import get_cart
+
+    g.cart = await get_cart(g.s)