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/public/templates/emails/contact_email.txt

@@ -1,10 +1,6 @@
-New contact form inquiry — {{ brand_name|default:"Monica Dhillon" }}
+New contact form inquiry — {{ brand_name|default:"Your Company" }}
 
-Name: {{ name }}
 Email: {{ email }}
-Phone: {{ phone }}
-Address:
-{{ address }}
 
 Message:
 {{ message }}
@@ -12,6 +8,6 @@ Message:
 {% if portal_url %}View in portal: {{ portal_url }}
 {% endif %}
 —
-{{ brand_name|default:"Monica Dhillon" }}
-{{ site_url|default:"https://mkdrealtor.com" }}
+{{ brand_name|default:"Your Company" }}
+{{ site_url|default:"" }}
 {% if brand_tagline %}{{ brand_tagline }}{% endif %}