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.

.env.example

@@ -88,6 +88,7 @@ POS_API_TOKEN=
 
 # Shipping — FEATURE_SHIPPING (requires shop). EasyPost optional; CSV export always works.
 EASYPOST_API_KEY=
+EASYPOST_WEBHOOK_SECRET=
 SHIP_FROM_LINE1=
 SHIP_FROM_CITY=
 SHIP_FROM_STATE=