Add shopper accounts, reviews, tracking, and seed_demo (#9)

Closes #9. Shop-gated buyer accounts, purchase reviews, Stripe customer ids, shipment tracking, slim public contact form, and a template-neutral seed_demo command.

site/accounts/services.py

@@ -0,0 +1,22 @@
+from accounts.models import CustomerProfile
+
+
+def get_customer_profile(user) -> CustomerProfile:
+    profile, _created = CustomerProfile.objects.get_or_create(user=user)
+    return profile
+
+
+def claim_orders_for_user(user) -> int:
+    """Attach guest orders that used this email so history/reviews work after signup."""
+    from django.apps import apps
+
+    if not apps.is_installed("shop"):
+        return 0
+    from shop.models import Order
+
+    email = (user.email or user.username or "").strip()
+    if not email:
+        return 0
+    return Order.objects.filter(user__isnull=True, email__iexact=email).update(
+        user=user
+    )