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/pos_sync/hooks.py

@@ -0,0 +1,35 @@
+from core.registry import (
+    register_dashboard_collector,
+    register_dispatcher,
+    register_feature,
+    register_portal_nav,
+)
+
+
+def register() -> None:
+    register_feature("pos_sync")
+    register_portal_nav(
+        section="pos_sync",
+        label="POS sync",
+        url_name="pos_sync:event_list",
+        group="Retail",
+        order=40,
+    )
+    register_dashboard_collector(_dashboard)
+    register_dispatcher(_dispatch)
+
+
+def _dashboard(request) -> dict:
+    from pos_sync.models import SyncEvent
+
+    return {
+        "pos_failed_syncs": SyncEvent.objects.filter(
+            status=SyncEvent.Status.FAILED
+        ).count()
+    }
+
+
+def _dispatch() -> int:
+    from pos_sync.services import dispatch_pending_outbound
+
+    return dispatch_pending_outbound()