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/models.py

@@ -93,7 +93,9 @@ class FamilyMembership(models.Model):
         verbose_name = _("family membership")
         verbose_name_plural = _("family memberships")
         ordering = ["family", "role", "member__last_name", "member__first_name"]
-        unique_together = ("family", "member")
+        constraints = [
+            models.UniqueConstraint(fields=["family", "member"], name="unique_member_per_family"),
+        ]
 
     def __str__(self):
         return f"{self.family} - {self.member} ({self.get_role_display()})"