🗃️(models) remove 'secret' field from mailbox model

We remove 'secret' field, as it won't be of use in interactions
between la Régie and dimail. Régie credentials will be stored and used
as project variable.

src/backend/mailbox_manager/models.py

@@ -26,7 +26,6 @@ class MailDomain(BaseModel):
         default=MailDomainStatusChoices.PENDING,
         choices=MailDomainStatusChoices.choices,
     )
-    secret = models.CharField(_("secret"), max_length=255, null=True, blank=True)
 
     class Meta:
         db_table = "people_mail_domain"
@@ -139,15 +138,19 @@ class Mailbox(BaseModel):
         return f"{self.local_part!s}@{self.domain.name:s}"
 
     def clean(self):
-        """Mailboxes can be created only on enabled domains, with a set secret."""
+        """
+        Mailboxes can only be created on enabled domains.
+        Also, mail-provisioning API credentials must be set for dimail to allow auth.
+        """
         if self.domain.status != MailDomainStatusChoices.ENABLED:
             raise exceptions.ValidationError(
                 "You can create mailbox only for a domain enabled"
             )
 
-        if not self.domain.secret:
+        # Won't be able to query user token if MAIL_PROVISIONING_API_CREDENTIALS are not set
+        if not settings.MAIL_PROVISIONING_API_CREDENTIALS:
             raise exceptions.ValidationError(
-                "Please configure your domain's secret before creating any mailbox."
+                "Please configure MAIL_PROVISIONING_API_CREDENTIALS before creating any mailbox."
             )
 
     def save(self, *args, **kwargs):