✨(models) impose uniqueness on display name, to match ox's constraint

OpenXchange's primary key is display name (= first name + last name).
It must be unique in the domain's context. We don't have context info
but we can impose uniqueness by domain.

src/backend/mailbox_manager/migrations/0026_alter_mailbox_unique_together_and_more.py

@@ -0,0 +1,25 @@
+# Generated by Django 5.2.5 on 2025-09-08 12:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0025_alter_mailbox_secondary_email'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='mailbox',
+            unique_together=set(),
+        ),
+        migrations.AddConstraint(
+            model_name='mailbox',
+            constraint=models.UniqueConstraint(fields=('local_part', 'domain'), name='unique_username'),
+        ),
+        migrations.AddConstraint(
+            model_name='mailbox',
+            constraint=models.UniqueConstraint(fields=('first_name', 'last_name', 'domain'), name='unique_ox_display_name'),
+        ),
+    ]