refactor(accounts): UserInstance's uniqueness is a UniqueConstraint

Django documents unique_together as likely to be deprecated in favour of
Meta.constraints; the rule is the same (one row per user and VM).

accounts/migrations/0012_user_instance_constraint.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.2.18 on 2026-11-06 07:57
+
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0011_quota_help'),
+        ('instances', '0014_instancetombstone_host'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='userinstance',
+            unique_together=set(),
+        ),
+        migrations.AddConstraint(
+            model_name='userinstance',
+            constraint=models.UniqueConstraint(fields=('user', 'instance'), name='unique_user_instance'),
+        ),
+    ]

accounts/models.py

@@ -27,7 +27,7 @@ class UserInstance(models.Model):
         }
 
     class Meta:
-        unique_together = ["user", "instance"]
+        constraints = [models.UniqueConstraint(fields=["user", "instance"], name="unique_user_instance")]
 
 
 class UserSSHKey(models.Model):