✨(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/models.py

@@ -303,7 +303,15 @@ class Mailbox(AbstractBaseUser, BaseModel):
         db_table = "people_mail_box"
         verbose_name = _("Mailbox")
         verbose_name_plural = _("Mailboxes")
-        unique_together = ("local_part", "domain")
+        constraints = [
+            models.UniqueConstraint(
+                fields=["local_part", "domain"], name="unique_username"
+            ),
+            models.UniqueConstraint(
+                fields=["first_name", "last_name", "domain"],
+                name="unique_ox_display_name",
+            ),
+        ]
         ordering = ["-created_at"]
 
     def __str__(self):