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/templates/pos_sync/list.html

@@ -0,0 +1,22 @@
+{% extends "portal_base.html" %}
+{% block title %}POS sync · Portal{% endblock %}
+{% block topbar_title %}POS sync{% endblock %}
+{% block portal_content %}
+<p class="muted">Inbound POS sales decrement shop stock. Online sales queue outbound reserves.</p>
+<table class="table">
+  <thead><tr><th>When</th><th>Direction</th><th>SKU</th><th>Qty</th><th>Status</th></tr></thead>
+  <tbody>
+    {% for event in events %}
+    <tr>
+      <td>{{ event.created_at }}</td>
+      <td>{{ event.get_direction_display }}</td>
+      <td>{{ event.sku }}</td>
+      <td>{{ event.quantity }}</td>
+      <td>{{ event.get_status_display }}</td>
+    </tr>
+    {% empty %}
+    <tr><td colspan="5" class="empty-state">No sync events yet.</td></tr>
+    {% endfor %}
+  </tbody>
+</table>
+{% endblock %}