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/management/commands/import_members_csv.py

@@ -35,7 +35,6 @@ class Command(BaseCommand):
                 f"Members created: {result.created_members}. "
                 f"Members updated: {result.updated_members}. "
                 f"Users created: {result.created_users}. "
-                f"Clubs created: {result.created_clubs}. "
                 f"Memberships created: {result.created_memberships}. "
                 f"Memberships updated: {result.updated_memberships}. "
                 f"Rows skipped: {result.skipped_rows}."