feat(tours): allow nullable category relations

Updated `originalmedia` and `scene` models to allow their `category`
fields to be nullable. This adjustment permits entries without a
specific category, enhancing flexibility in content organization. Upon
deletion of a category, related records will now have their category set
to null instead of being deleted, preventing unintended data loss.

Resolves issues with rigid category assignments and improves data
resilience.

quackscape/tours/migrations/0015_alter_originalmedia_category_alter_scene_category.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.0.3 on 2024-03-16 06:30
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tours', '0014_teleportelement_destination_z'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='originalmedia',
+            name='category',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='media', to='tours.category'),
+        ),
+        migrations.AlterField(
+            model_name='scene',
+            name='category',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='scenes', to='tours.category'),
+        ),
+    ]