feat: season-scope club memberships and convert to UniqueConstraint

ClubMembership is now tied to a Season (plus status / fee_status / sign-up
dates) and unique per (club, member, season). The CSV importer attaches
each membership to the club's current season (Season.get_current), skipping
the row with a clear error when none exists; the now-unreachable
"clubs created" bookkeeping is removed.

Register SeasonAdmin and rebuild ClubMembershipAdmin for the new fields, and
add an admin smoke test that asserts every model in authentication/club/
members/teams is registered and its changelist + add pages load.

Convert every unique_together to a Meta UniqueConstraint (Season,
ClubMembership, FamilyMembership) per Django's guidance. Regenerate
migrations. club, members, and importer stay at 100% coverage.

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

members/migrations/0002_alter_familymembership_unique_together_and_more.py

@@ -0,0 +1,21 @@
+# Generated by Django 6.0.6 on 2026-07-12 14:12
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('members', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='familymembership',
+            unique_together=set(),
+        ),
+        migrations.AddConstraint(
+            model_name='familymembership',
+            constraint=models.UniqueConstraint(fields=('family', 'member'), name='unique_member_per_family'),
+        ),
+    ]