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/0006_alter_order_number.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-07-12 20:14
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('shop', '0005_product_staff_role_cartitem_order'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='order',
+            name='number',
+            field=models.CharField(blank=True, max_length=255, verbose_name='number'),
+        ),
+    ]