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/shop/templates/shop/detail.html

@@ -0,0 +1,20 @@
+{% extends "base.html" %}
+{% block title %}{{ product.name }} · {{ SITE_NAME }}{% endblock %}
+{% block content %}
+<section class="section section-lg">
+  <div class="container">
+    <p><a href="{% url 'shop:list' %}">← Shop</a></p>
+    <h1>{{ product.name }}</h1>
+    <p class="muted">{{ product.price }} {{ product.currency|upper }} · {{ product.sku }}</p>
+    <div>{{ product.description|linebreaks }}</div>
+    {% if available is not None %}
+    <p>{{ available }} in stock</p>
+    {% endif %}
+    <form method="post" action="{% url 'shop:cart_add' product.slug %}">
+      {% csrf_token %}
+      <label>Qty <input name="quantity" type="number" min="1" value="1"></label>
+      <button class="button button-primary" type="submit">Add to cart</button>
+    </form>
+  </div>
+</section>
+{% endblock %}