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

site/public/context_processors.py

@@ -63,6 +63,15 @@ def tianji_tracking(request):
         nav_section = "social_accounts"
     elif namespace == "social":
         nav_section = "social"
+    elif namespace == "shop_portal":
+        if url_name.startswith("product"):
+            nav_section = "shop_products"
+        elif url_name.startswith("order"):
+            nav_section = "shop_orders"
+        elif url_name == "sales":
+            nav_section = "shop_sales"
+    elif namespace == "shop":
+        nav_section = "shop"
 
     return {
         "tianji_enabled": getattr(settings, "TIANJI_ENABLED", False)