feat(tenancy): add Season current-season lookup and year name

Complete the Season model:

- name property renders the start/end years as a "YY-YY" label (e.g.
  "25-26") via strftime %y, and __str__ now returns it.
- get_current(date) returns the active club's season covering the given
  date (today by default), inclusive of both boundaries, scoped through
  the tenant queryset so it never crosses clubs.

Rename the tenant queryset's current() to current_club() for clarity and
update callers/tests. Cover the new behaviour; tenancy modules stay at
100%.

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

clubmanager/base.py

@@ -13,7 +13,7 @@ class TenantQuerySet(models.QuerySet):
     def for_club(self, club: Club):
         return self.filter(club=club)
 
-    def current(self):
+    def current_club(self):
         return self.filter(club=require_current_club())