Add read-only admin functionality and enhance billing models in admin interface

backend/igny8_core/admin/apps.py

@@ -1,8 +1,43 @@
+from django.contrib import admin
 from django.contrib.admin.apps import AdminConfig
 
 
+class ReadOnlyAdmin(admin.ModelAdmin):
+    """Generic read-only admin for system tables."""
+
+    def has_add_permission(self, request):
+        return False
+
+    def has_change_permission(self, request, obj=None):
+        return False
+
+    def has_delete_permission(self, request, obj=None):
+        return False
+
+
+def _safe_register(model, model_admin):
+    try:
+        admin.site.register(model, model_admin)
+    except admin.sites.AlreadyRegistered:
+        pass
+
+
 class Igny8AdminConfig(AdminConfig):
     default_site = 'igny8_core.admin.site.Igny8AdminSite'
     name = 'django.contrib.admin'
 
+    def ready(self):
+        super().ready()
+        # Register Django internals in admin (read-only where appropriate)
+        from django.contrib.admin.models import LogEntry
+        from django.contrib.auth.models import Group, Permission
+        from django.contrib.contenttypes.models import ContentType
+        from django.contrib.sessions.models import Session
+
+        _safe_register(LogEntry, ReadOnlyAdmin)
+        _safe_register(Permission, admin.ModelAdmin)
+        _safe_register(Group, admin.ModelAdmin)
+        _safe_register(ContentType, ReadOnlyAdmin)
+        _safe_register(Session, ReadOnlyAdmin)
+