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

events/migrations/0011_location_is_home_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.6 on 2026-08-04 09:37
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0017_club_season_duration_months_club_season_start'),
+        ('events', '0010_attendance_showed_up'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='location',
+            name='is_home',
+            field=models.BooleanField(default=False, help_text="The club's own ground, set from the control panel -- lets an event's location tell a home game from an away one.", verbose_name='home location'),
+        ),
+        migrations.AddConstraint(
+            model_name='location',
+            constraint=models.UniqueConstraint(condition=models.Q(('is_home', True)), fields=('club',), name='unique_home_location_per_club'),
+        ),
+    ]