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

management/templates/management/opponent_form.html

@@ -0,0 +1,31 @@
+{% extends "management/base.html" %}
+{% load i18n lucide ui %}
+
+{% block heading %}{% if update_view %}{% blocktrans %}Edit {{ object }}{% endblocktrans %}{% else %}{% trans "New opponent" %}{% endif %}{% endblock heading %}
+
+{% block panel %}
+    <div class="card w-full bg-base-100 shadow">
+        <div class="card-body">
+            <form method="post" enctype="multipart/form-data">
+                {% csrf_token %}
+
+                {% for error in form.non_field_errors %}
+                    <div class="alert alert-error my-2">
+                        <span>{{ error }}</span>
+                    </div>
+                {% endfor %}
+
+                <div class="grid grid-cols-1 md:grid-cols-2 gap-4">
+                    {% for field in form %}
+                        {% form_field field %}
+                    {% endfor %}
+                </div>
+
+                <div class="card-actions justify-start pt-2 mt-2">
+                    <a class="btn btn-outline btn-neutral gap-2" href="{% url "management:opponent_list" %}">{% lucide "arrow-left" size=16 %} {% trans "Cancel" %}</a>
+                    <button class="btn btn-primary gap-2" type="submit">{% lucide "save" size=16 %} {% trans "Save" %}</button>
+                </div>
+            </form>
+        </div>
+    </div>
+{% endblock panel %}