Add shopper accounts, reviews, tracking, and seed_demo (#10)

## Summary
- Closes #9. Port shop-gated buyer accounts (register/login/profile, password reset, order history), purchase-gated reviews, and Stripe customer ids. Cards stay on Stripe.
- Slim public contact form to email + interest + message. Shipping tracking status, scan events, paste-in numbers, and EasyPost webhook when FEATURE_SHIPPING is on.
- Add template-neutral `seed_demo` (generic SKUs, demo shoppers, guest orders, tracking stubs). Prod refused. `--reset` deletes tagged demo rows after invoices.

## Test plan
- [ ] `cd site && uv run python manage.py test` (173 tests passed locally)
- [ ] Shop on: `/account/register`, login, profile, password reset, claim guest orders by email
- [ ] Shop off: `/account/` URLs absent; Sign in / Account links hidden
- [ ] Checkout as logged-in buyer stores `Order.user` and Stripe customer id; product review only after paid purchase
- [ ] Shipping on: portal paste-in tracking + EasyPost webhook updates status; shipping off: orders still work without tracking UI
- [ ] `seed_demo` refused in prod; `--reset` rebuilds tagged catalog/shoppers without deleting staff invoices

Reviewed-on: #10

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
+    )