Add shop, POS sync, event ticketing, and shipping catalog apps (#6)

## Summary
- Closes #5
- Optional `shop`, `pos_sync`, `events`, and `shipping` apps gated by `FEATURE_*` flags
- Deps match the catalog: shop/events need email + Stripe; POS/shipping need shop
- Portal inventory, POS webhooks, capacity tickets, EasyPost/Pirate Ship shipping

## Test plan
- [ ] `manage.py test` (152 passed locally)
- [ ] Shop cart + paid order decrements stocked inventory
- [ ] POS inbound webhook decrements SKU; paid order queues outbound reserve
- [ ] Event capacity blocks overbook; paid order emails ticket codes
- [ ] Shipping stub label + Pirate Ship CSV of unshipped paid orders
- [ ] `validate-env.sh` rejects shop without email/payments, POS/shipping without shop

Reviewed-on: #6

.env.prod.example

@@ -32,6 +32,10 @@ FEATURE_BLOG=false
 FEATURE_PAYMENTS=false
 FEATURE_SOCIAL=false
 FEATURE_SOCIAL_AI=false
+FEATURE_SHOP=false
+FEATURE_POS_SYNC=false
+FEATURE_EVENTS=false
+FEATURE_SHIPPING=false
 
 # Shared external Postgres (10.0.0.230)
 DATABASE_URL=postgres://westfarn:replace-db-password@10.0.0.230:5432/client_site
@@ -63,6 +67,21 @@ STRIPE_PUBLISHABLE_KEY=
 STRIPE_WEBHOOK_SECRET=
 STRIPE_CURRENCY=usd
 
+# Required when FEATURE_SHOP=true (also requires FEATURE_EMAIL_SMS + FEATURE_PAYMENTS)
+# SHOP_PRINT_QUEUE_LIMIT_MINUTES=0
+
+# Required when FEATURE_POS_SYNC=true in beta/prod (also requires FEATURE_SHOP)
+POS_WEBHOOK_SECRET=
+POS_API_BASE_URL=
+POS_API_TOKEN=
+
+# Optional when FEATURE_SHIPPING=true (also requires FEATURE_SHOP)
+EASYPOST_API_KEY=
+SHIP_FROM_LINE1=
+SHIP_FROM_CITY=
+SHIP_FROM_STATE=
+SHIP_FROM_ZIP=
+
 # Required when FEATURE_SOCIAL=true
 META_APP_ID=
 META_APP_SECRET=