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/models.py

@@ -0,0 +1,118 @@
+from decimal import Decimal
+
+from django.db import models
+from django.urls import reverse
+from django.utils.text import slugify
+
+from core.models import TimeStampedModel, UUIDPrimaryKeyModel
+
+
+class Product(UUIDPrimaryKeyModel, TimeStampedModel):
+    class Fulfillment(models.TextChoices):
+        STOCKED = "stocked", "On-hand stock"
+        MADE_TO_ORDER = "made_to_order", "Made to order"
+
+    name = models.CharField(max_length=200)
+    slug = models.SlugField(max_length=220, unique=True)
+    sku = models.CharField(max_length=64, unique=True)
+    description = models.TextField(blank=True)
+    price = models.DecimalField(max_digits=10, decimal_places=2)
+    currency = models.CharField(max_length=8, default="usd")
+    fulfillment = models.CharField(
+        max_length=16,
+        choices=Fulfillment.choices,
+        default=Fulfillment.STOCKED,
+    )
+    stock_qty = models.IntegerField(default=0)
+    print_minutes = models.PositiveIntegerField(
+        default=0,
+        help_text="Estimated print time per unit (made-to-order).",
+    )
+    filament_grams = models.PositiveIntegerField(default=0)
+    is_published = models.BooleanField(default=False)
+    track_inventory = models.BooleanField(default=True)
+
+    class Meta:
+        ordering = ["name"]
+
+    def __str__(self) -> str:
+        return f"{self.name} ({self.sku})"
+
+    def get_absolute_url(self) -> str:
+        return reverse("shop:detail", kwargs={"slug": self.slug})
+
+    @property
+    def amount_cents(self) -> int:
+        return int((self.price * Decimal("100")).quantize(Decimal("1")))
+
+    def save(self, *args, **kwargs):
+        if not self.slug:
+            base = slugify(self.name)[:200] or "product"
+            slug = base
+            n = 2
+            while Product.objects.filter(slug=slug).exclude(pk=self.pk).exists():
+                slug = f"{base}-{n}"
+                n += 1
+            self.slug = slug
+        if not self.sku:
+            self.sku = (self.slug or "sku").upper().replace("-", "")[:64]
+        super().save(*args, **kwargs)
+
+
+class Order(UUIDPrimaryKeyModel, TimeStampedModel):
+    class Status(models.TextChoices):
+        DRAFT = "draft", "Draft"
+        OPEN = "open", "Open"
+        PAID = "paid", "Paid"
+        FULFILLED = "fulfilled", "Fulfilled"
+        CANCELLED = "cancelled", "Cancelled"
+
+    number = models.CharField(max_length=32, unique=True)
+    email = models.EmailField()
+    customer_name = models.CharField(max_length=200, blank=True)
+    status = models.CharField(
+        max_length=16, choices=Status.choices, default=Status.DRAFT
+    )
+    amount = models.DecimalField(max_digits=10, decimal_places=2, default=Decimal("0"))
+    currency = models.CharField(max_length=8, default="usd")
+    shipping_address = models.JSONField(default=dict, blank=True)
+    stripe_checkout_session_id = models.CharField(max_length=255, blank=True)
+    hosted_checkout_url = models.URLField(blank=True)
+    paid_at = models.DateTimeField(null=True, blank=True)
+    notes = models.TextField(blank=True)
+
+    class Meta:
+        ordering = ["-created_at"]
+
+    def __str__(self) -> str:
+        return f"{self.number} · {self.email} · {self.amount}"
+
+    @property
+    def amount_cents(self) -> int:
+        return int((self.amount * Decimal("100")).quantize(Decimal("1")))
+
+
+class OrderItem(UUIDPrimaryKeyModel, TimeStampedModel):
+    order = models.ForeignKey(Order, on_delete=models.CASCADE, related_name="items")
+    product = models.ForeignKey(
+        Product,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="order_items",
+    )
+    name = models.CharField(max_length=200)
+    sku = models.CharField(max_length=64)
+    quantity = models.PositiveIntegerField(default=1)
+    unit_price = models.DecimalField(max_digits=10, decimal_places=2)
+    print_minutes = models.PositiveIntegerField(default=0)
+
+    class Meta:
+        ordering = ["created_at"]
+
+    def __str__(self) -> str:
+        return f"{self.quantity}× {self.name}"
+
+    @property
+    def line_total(self) -> Decimal:
+        return self.unit_price * self.quantity