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

README.md

@@ -23,6 +23,10 @@ separate Django apps. A flag that is off keeps the app out of
 | Payments (Stripe) | `FEATURE_PAYMENTS` | `payments` (requires email/SMS) |
 | Social consolidation | `FEATURE_SOCIAL` | `social` |
 | AI social generator | `FEATURE_SOCIAL_AI` | `social_ai` (requires social) |
+| E-commerce storefront | `FEATURE_SHOP` | `shop` (requires email/SMS + payments) |
+| POS API sync | `FEATURE_POS_SYNC` | `pos_sync` (requires shop) |
+| Event ticketing | `FEATURE_EVENTS` | `events` (requires email/SMS + payments) |
+| Shipping automation | `FEATURE_SHIPPING` | `shipping` (requires shop) |
 
 ## New client