Rework Teams/Positions access control and add full Locations/Opponents CRUD

Team managers/coaches now only see their own teams, can't create teams,
and can view (but not edit) positions -- admins keep full rights.
Locations and Opponents move from read-only stubs to full CRUD, gated
to admins and management-position staff, with a country dropdown
(django-countries) instead of free text. Also: the team list shows
player/staff counts, and deleting a news item's main photo promotes
another one instead of leaving the item without one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R1gj3J1QPfP38XWpnpbFpy

club/mixins.py

@@ -1,7 +1,7 @@
 from django.contrib.auth.mixins import LoginRequiredMixin, UserPassesTestMixin
 from django.http import Http404
 
-from .services.access import can_add_news, can_edit_news, can_publish_news, has_management_access, is_club_admin, teams_managed_by
+from .services.access import can_add_news, can_edit_news, can_publish_news, has_management_access, is_club_admin, is_coach_manager, teams_managed_by
 
 
 class ClubStaffRequiredMixin(LoginRequiredMixin, UserPassesTestMixin):
@@ -51,6 +51,16 @@ class TeamManagerRequiredMixin(ClubStaffRequiredMixin):
         return teams_managed_by(user, club).filter(pk=self.get_team().pk).exists()
 
 
+class ManagementPositionRequiredMixin(ClubStaffRequiredMixin):
+    """ADMIN, or anyone with a current-season *management*-position
+    StaffAssignment on any team -- unlike ``TeamManagerRequiredMixin``, the
+    entity here (Location, Opponent, ...) isn't scoped to one team, so "manager
+    of this team" doesn't apply; any management position qualifies."""
+
+    def test_func(self):
+        return is_club_admin(self.request.user, self.request.club) or is_coach_manager(self.request.user, self.request.club)
+
+
 class NewsAuthorRequiredMixin(ClubStaffRequiredMixin):
     """ADMIN, EDITOR, or a current-season coach_manager -- who's trusted to
     author club content in the first place (creating a draft)."""