feat(club): archive clubs instead of deleting them

Add Club.archived_at with active()/archived() managers, archive() and
restore(). An archived club stops resolving in ClubTenantMiddleware, so its
subdomain behaves as unknown — archiving is a real deactivation, not a
cosmetic flag — while every row it owns is retained.

There is no hard-delete path, deliberately. A club with any data cannot be
deleted anyway (ClubMembership PROTECTs its Season, and the shop chain
PROTECTs more), and invoices generally must be kept.

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

club/migrations/0010_club_archived_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-07-13 13:15
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0009_clubrole'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='archived_at',
+            field=models.DateTimeField(blank=True, help_text='Archived clubs stop resolving on their subdomain, but their data is retained.', null=True, verbose_name='archived at'),
+        ),
+    ]