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

pyproject.toml

@@ -6,6 +6,7 @@ dependencies = [
     "dj-database-url>=3.1.2",
     "django>=6.0.6",
     "django-allauth[mfa]>=65.18.0",
+    "django-countries>=9.0.0",
     "django-lucide",
     "django-phonenumber-field[phonenumbers]>=8.4.0",
     "django-redis>=7.0.0",