Use `SET_NULL` rather than `CASCADE` for SubsidyPayment attachments

scipost_django/finances/migrations/0030_auto_20240312_1822.py

+# Generated by Django 3.2.18 on 2024-03-12 17:22
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('finances', '0029_subsidyattachment_git_url_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='subsidypayment',
+            name='invoice',
+            field=models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='invoice_for', to='finances.subsidyattachment'),
+        ),
+        migrations.AlterField(
+            model_name='subsidypayment',
+            name='proof_of_payment',
+            field=models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='proof_of_payment_for', to='finances.subsidyattachment'),
+        ),
+    ]

scipost_django/finances/models.py

@@ -150,14 +150,14 @@ class SubsidyPayment(models.Model):
     date_scheduled = models.DateField()
     invoice = models.OneToOneField(
         "finances.SubsidyAttachment",
-        on_delete=models.CASCADE,
+        on_delete=models.SET_NULL,
         blank=True,
         null=True,
         related_name="invoice_for",
     )
     proof_of_payment = models.OneToOneField(
         "finances.SubsidyAttachment",
-        on_delete=models.CASCADE,
+        on_delete=models.SET_NULL,
         blank=True,
         null=True,
         related_name="proof_of_payment_for",