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_identity.py

@@ -0,0 +1,34 @@
+"""
+Cart identity resolution — shared across all apps that need to know
+who the current cart owner is (user_id or anonymous session_id).
+"""
+from __future__ import annotations
+
+import secrets
+from typing import TypedDict, Optional
+
+from quart import g, session as qsession
+
+
+class CartIdentity(TypedDict):
+    user_id: Optional[int]
+    session_id: Optional[str]
+
+
+def current_cart_identity() -> CartIdentity:
+    """
+    Decide how to identify the cart:
+
+    - If user is logged in -> use user_id (and ignore session_id)
+    - Else -> generate / reuse an anonymous session_id stored in Quart's session
+    """
+    user = getattr(g, "user", None)
+    if user is not None and getattr(user, "id", None) is not None:
+        return {"user_id": user.id, "session_id": None}
+
+    sid = qsession.get("cart_sid")
+    if not sid:
+        sid = secrets.token_hex(16)
+        qsession["cart_sid"] = sid
+
+    return {"user_id": None, "session_id": sid}