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

rosterchief/settings.py

@@ -54,6 +54,7 @@ INSTALLED_APPS = [
     # that page raises TemplateSyntaxError.
     "django.contrib.humanize",
     "phonenumber_field",
+    "django_countries",
     "lucide",
     # Auth: allauth deliberately WITHOUT django.contrib.sites — it is optional in
     # allauth 65+, and ARCHITECTURE.md §2.4 rejects the Sites framework (Club is
@@ -188,6 +189,7 @@ TEMPLATES = [
                 "club.context_processors.branding",
                 "features.context_processors.maintenance",
                 "management.context_processors.is_admin",
+                "management.context_processors.management_position",
                 "management.context_processors.management_link",
                 "management.context_processors.active_nav_section",
                 "management.context_processors.news_permissions",