feat(shop): cart, order, invoice and discounts

Build out the shop domain: Cart/CartItem (one open cart per user per club),
Order/OrderLine, Discount/AppliedDiscount, Payment and Invoice.

Order and Invoice allocate a per-club, per-year sequential number
(ORD-<year>-<seq> / INV-<year>-<seq>) via shared helpers, retrying on collision
with the (club, number) unique constraint as the source of truth.

Fixes found while testing:
- Invoice had no number generator, so a second invoice in a club collided on
  the empty string and could never be created.
- AppliedDiscount printed a "%" suffix even for fixed-amount discounts, and had
  no (order, discount) uniqueness, so a discount could be applied twice.

Every model validates its club-scoped FKs (product/team/discount/order/season/
staff_role) against the owning club, and Member FKs against club membership.
Register all models in the admin, with FK dropdowns scoped to the owning club.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shop/migrations/0004_cart.py

@@ -0,0 +1,32 @@
+# Generated by Django 6.0.6 on 2026-07-12 20:02
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0008_alter_clubmembership_unique_together_and_more'),
+        ('shop', '0003_alter_product_options_and_more'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Cart',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('status', models.CharField(choices=[('open', 'Open'), ('checked_out', 'Checked out'), ('abandoned', 'Abandoned')], default='open', max_length=255, verbose_name='status')),
+                ('club', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='club.club')),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='carts', to=settings.AUTH_USER_MODEL, verbose_name='user')),
+            ],
+            options={
+                'verbose_name': 'cart',
+                'verbose_name_plural': 'carts',
+                'constraints': [models.UniqueConstraint(condition=models.Q(('status', 'open')), fields=('club', 'user'), name='unique_open_cart_per_user_per_club')],
+            },
+        ),
+    ]