feat(tenancy): resolve active club from request subdomain

Add row-based multi-tenancy plumbing keyed on Club as the tenant root:

- ClubTenantMiddleware maps the request's subdomain to a Club by slug,
  storing it on request.club and in a contextvar so service-layer code
  and management commands can read it via get_current_club(). Resolution
  honours CLUBMANAGER_BASE_DOMAIN (e.g. ajax-united.clubmanager.app),
  falling back to generic slug.example.com hosts, and ignores the bare
  base domain, www, and unknown slugs.
- Club gains a unique slug (auto-derived from name on save) plus a
  ClubManager.current() accessor for the active tenant.
- ClubScopedModel gets a TenantQuerySet (.for_club()/.current()) and
  auto-fills club from the active context on save.

Contextvar helpers live in club.tenancy; Club is imported lazily there
and in clubmanager.base to avoid an import cycle with club.models.

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

club/migrations/0006_club_slug_season.py

@@ -0,0 +1,33 @@
+# Generated by Django 6.0.6 on 2026-07-12 13:11
+
+import django.db.models.deletion
+import uuid
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0005_alter_clubmembership_member'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='slug',
+            field=models.SlugField(blank=True, help_text='Drives subdomain / path resolution (e.g. ajax-united.clubmanager.app).', max_length=255, unique=True, verbose_name='slug'),
+        ),
+        migrations.CreateModel(
+            name='Season',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('start_date', models.DateField(verbose_name='start date')),
+                ('end_date', models.DateField(verbose_name='end date')),
+                ('club', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='club.club')),
+            ],
+            options={
+                'verbose_name': 'season',
+                'verbose_name_plural': 'seasons',
+            },
+        ),
+    ]